How to check if text was written by AI, and what the score means

By the end of this guide you will have run a piece of text through an AI content detector, read the result sensibly, gathered a second kind of evidence, and decided what to do next without treating a score as proof. A detector estimates whether writing looks like it came from a language model. It does not know who wrote the words.

If you want to check whether text was written by AI, the practical choice is between a detector built for teachers and publishers, a writing suite that includes a detector, and a service that also checks images and audio. This guide compares six tools from our Writing category and a neighbouring one, and shows how to use a score as a starting point for a conversation rather than a verdict.

Key takeaways

• Paste or upload the full text into a detector, read the document score and the sentence highlights, then look for other evidence before you decide anything.
• The main decision is the type of tool: a dedicated detector (GPTZero, Originality.ai), a writing suite with a detector (QuillBot, Grammarly), or a multi-format checker (AI or Not).
• Detectors can be wrong in both directions. GPTZero itself says results should not be used alone to punish students.
• Free tiers are small: for example 10,000 words a month on GPTZero, or a one-time 10K words on Originality.ai.

What you need before you start

• The full text. Longer samples give more reliable results than a few sentences. GPTZero says accuracy rises with longer text.
• The context. Who wrote it, what the task was, and what rules apply to AI use (course policy, editorial policy or client contract).
• Other evidence. Drafts, version history, notes and earlier writing by the same person.
• An account, possibly. Check whether the detector you pick asks you to sign up before you scan.
• Time. A scan takes minutes. Weighing the evidence and having a fair conversation takes longer.
• Difficulty. Easy to run, hard to interpret.
• A fair process. Decide in advance what you will do with a result, and who reviews it.

Which kind of AI tool fits this job

Choose by who you are and what else you need to check.

A dedicated detector for education and publishing. GPTZero gives a score for the whole document plus sentence-by-sentence highlighting, with a Deep Scan that breaks down how each sentence affects the AI probability. Writing Reports for Google Docs show editing statistics and a replay of the writing process. Originality.ai scans for AI-generated writing and plagiarism, accepts pasted text, Word or PDF files or a URL, and highlights the sentences most likely to be AI-written.

A writing suite that includes a detector. QuillBot bundles an AI detector with a paraphraser, grammar checker and plagiarism checker, though plagiarism checking is a Premium feature. Grammarly lists plagiarism and AI-generated text detection, but these require a paid plan.

A multi-format checker. AI or Not detects AI in text, images, voice, music and video, returns a verdict with confidence scores, and has a REST API. Its free text checker accepts DOC, DOCX and TXT files.

A free tool on a rewriting site. Clever AI Humanizer is mainly a site for rewriting AI drafts, which is not what this guide covers, but it also offers a free AI detector that reads English and Spanish. Use the detector as a second opinion, not as a main source.

Step 1: Decide why you are checking and what you will do with the result

A clear purpose keeps the process fair.

1. Write down the question: "Does this need a conversation?" is a safer question than "Did they cheat?"
2. Check the policy that applies, such as a course rule, newsroom standard or client agreement.
3. Decide who will review a high score and what the next step is.
4. Decide what other evidence you will look at before acting.

Check it worked by being able to state the rule that would apply if AI use were confirmed.

Step 2: Collect the full text and any supporting material

The detector sees only what you give it.

1. Get the complete piece, not an excerpt. Remove headings, quotes and references if they are not the writer's own words.
2. Gather earlier drafts, version history, outlines or notes if they exist.
3. Find other writing by the same person for comparison, with their permission where needed.
4. If you are an author wanting to prove your process, consider tools that record it. GPTZero's Writing Reports and the Originality.ai Chrome extension both list a writing replay.

Check it worked by having the final text plus at least one other source of evidence.

Step 3: Choose a tool to check if text was written by AI

Pick one or two detectors, and note their limits.

1. For a teacher or student workflow, look at GPTZero, which lists a free plan with 10,000 words a month and up to 10,000 characters per scan.
2. For publishing or agency review, look at Originality.ai, which lists AI detection, plagiarism and readability checks.
3. If you already use a writing suite, check whether QuillBot or Grammarly includes a detector on your plan.
4. If you are checking other media too, look at AI or Not.
5. Remember that different detectors read text differently, and none is perfect.

Check it worked by noting the tool, its free limits and what it claims to detect.

Step 4: Run the scan

Get a score and the sentence-level detail.

1. Paste the text or upload the file. Originality.ai accepts Word or PDF files and URLs, and AI or Not accepts DOC, DOCX and TXT files.
2. Run the scan and wait for the report.
3. Read the document-level score and the highlighted sentences.
4. If the tool offers a deeper view, run it. GPTZero's Deep Scan shows a statistical breakdown, and Originality.ai's Deep Scan shows why text was flagged.
5. Save the report with the date and the tool name.

Check it worked by having a saved report with a score and highlights.

Step 5: Read the result with care

A number is not a finding.

1. Treat the score as a probability or confidence level, not as proof. AI or Not describes its results as confidence scores, not proof of authorship.
2. Look at which sentences are flagged. GPTZero notes that accuracy is lower at sentence level than document level.
3. Consider known weak spots: heavily edited AI text and procedural text can be misjudged, according to GPTZero.
4. Run a second detector if the stakes are high, and compare, but do not average them into a verdict.

Check it worked by being able to explain in plain words what the score does and does not tell you.

Step 6: Look for other evidence

A fair decision needs more than a detector.

1. Compare the text with the writer's earlier work: vocabulary, structure and typical mistakes.
2. Check version history or drafts for a natural writing process.
3. Verify the facts and citations. AI text can include references that do not exist.
4. Ask the writer to explain the content, talk through their reasoning, or write a short section again in person.
5. Run a plagiarism check if copying is a concern. GPTZero's paid plans, Originality.ai, QuillBot Premium and Grammarly's paid plans list plagiarism checks.

Check it worked by having at least two independent pieces of evidence, one of them not a detector.

Step 7: Decide, document and respond fairly

Handle the outcome in a way you could defend.

1. If the evidence is weak, say so and close the matter, or ask for a draft history next time.
2. If the evidence points to undisclosed AI use, talk to the writer first and give them a chance to respond.
3. Record what you looked at and why you reached your decision.
4. Share your AI policy clearly so people know the rules in future.

Check it worked by having a written note that another reviewer would find reasonable.

Tools for this job compared

Tool	Good fit for	Pricing	Stated limits
GPTZero	Document and sentence-level detection for educators (web, Chrome extension, API)	Freemium, paid from $10/mo (checked 24 Oct 2026)	Free: 10,000 words a month and 7 scans per hour; can misclassify in both directions
Originality.ai	AI and plagiarism checks for publishers and agencies (web, Chrome and Firefox extensions, API)	Freemium, paid from $12.95/mo (checked 24 Oct 2026)	Free plan is a one-time 10K words; API access listed as Enterprise only
QuillBot	Writing suite with an AI detector (web, mobile app)	Freemium	Plagiarism checking is Premium with 25,000 words per month; Premium usable on 10 devices at a time
AI or Not	Checking text, images, audio and video (web, API, Chrome extension)	Freemium, paid from $5/mo (checked 24 Oct 2026)	Free text checker accepts DOC, DOCX and TXT; results are confidence scores, not proof
Clever AI Humanizer	A free second-opinion detector for English and Spanish (web)	Free	Detector reads English and Spanish only; the site is mainly a rewriting tool, so treat its detector as a second opinion
Grammarly	Writing help in many apps, with detection on paid plans (web, iOS, Android)	Freemium, paid from $12/mo (checked 24 Oct 2026)	Plagiarism and AI text detection need a paid plan; Free includes 100 AI prompts a month

Prices and plan limits change, so confirm the current terms on each vendor's site before you subscribe.

Common mistakes to avoid

• Treating a score as proof. Detectors give probabilities and can be wrong. Fix it by using the result as one input and confirming with other evidence.
• Scanning a short snippet. Short samples give weaker results. Fix it by scanning the whole piece, excluding quotes and references.
• Relying on one detector. Different tools read text differently. Fix it by comparing at most two tools, and treating disagreement as a reason for caution.
• Skipping the conversation. Acting on a score alone can wrongly accuse someone. Fix it by asking the writer for drafts and an explanation before any decision.
• Having no policy. People cannot follow rules they have not seen. Fix it by publishing what AI use is allowed, how to disclose it and how concerns are handled.

How we put this together

This is an evidence-based guide built from each tool's listing in our directory, which is compiled from the vendor's own site and pricing page. It is not a review based on using the tools. We don't rank tools, and nobody paid to be included. Some outbound links may be affiliate links; see our disclosure.

Frequently asked questions

Is there a free way to check if text was written by AI?

Yes, within limits. GPTZero's Free plan allows 10,000 words a month, Originality.ai's Free plan includes a one-time 10K words, AI or Not has a free text checker, and Clever AI Humanizer lists a free detector. Paid features such as plagiarism checks usually need a subscription.

Can AI detectors be accurate?

They give useful signals, but they are not reliable enough to settle a case alone. GPTZero says results can misclassify text in both directions, that accuracy is lower at sentence level than document level, and that heavily edited AI text can be misjudged.

Can I prove that I wrote something myself?

You can show your process. GPTZero's Writing Reports for Google Docs show editing statistics and a replay of the writing process, and the Originality.ai Chrome extension lists a writing replay that can be shared as proof of authorship. Keeping drafts and notes also helps.

Which tool should a teacher use?

GPTZero lists teachers and students as its main audience, with Google Docs reports and LMS integrations. Originality.ai lists Google Classroom and Moodle support. Whichever you choose, use the score to start a conversation and follow your school's policy.

Compare more options in our Writing category.