How can a teacher tell if a student used ChatGPT?
Short answer: from the finished text alone, you usually can't. Not reliably, not fairly, and not in a way you could defend to a parent.
Cognity was started by a former elementary school teacher, and we still think about grading the way a teacher does. So this is the answer we would want if we were in the classroom, rather than the one that sells a product.
What you can notice, and why it isn't enough
Experienced teachers do notice things. A sudden change in voice. Vocabulary the student has never used in class. Paragraphs that are perfectly balanced but say nothing specific. Citations that look plausible and don't exist. These are real signals, and they are worth a conversation.
They are not evidence. A student who has been reading more, or who got help from an older sibling, or who is writing in their third language, produces exactly the same signals. If you accuse on the basis of "this doesn't sound like you," you will sometimes be right and you will sometimes be doing real damage to a student who did the work.
Why AI detectors don't solve this
Detectors estimate the probability that a piece of text was machine-generated. Three problems follow from that.
First, they are wrong often enough to matter. OpenAI shut down its own detector in 2023 for low accuracy. Studies since then have found that detectors flag non-native English writers at much higher rates than native speakers, because clear, formulaic English is exactly what both groups produce.
Second, a score is not a record. Even when a detector is right, it tells you nothing about what the student did. Did they paste a whole essay? Ask for an outline and write it themselves? Use AI to check grammar on their own draft? Those are three completely different situations that a detector scores the same way.
Third, a score puts you and the student on opposite sides of a table. You have a number; they have a denial. There is nowhere for that conversation to go.
MIT Sloan's teaching guidance says this plainly: detectors "don't work," and recommends process statements and staged assignments instead. That advice is right, and it points to the actual answer.
What works: look at the process, not the product
The question "did the student use AI?" is unanswerable from the text and, in 2026, almost beside the point. Most of them did, somewhere. The useful question is "how did the student work?" and that question has an answer, if the work happened somewhere that kept a record.
A record of the writing process shows you what a detector cannot. Which prompts the student typed. What the AI answered. What the student kept, changed, or rejected. How much of the final text came from their own keyboard. Whether they verified a claim or just pasted it. When a student's work is visible like this, you don't have to guess, and you don't have to accuse. You read the record and you talk about what you see.
This is the difference between AI detection and what we call AI visibility. Detection is an estimate about the output. Visibility is a record of the process.
How to get a record without becoming a surveillance officer
Three approaches, from lightest to most complete.
Process statements. Ask students to write two or three sentences on how they did the work, including what AI they used and for what. This costs nothing and shifts the norm from hiding AI use to declaring it. It relies on honesty, so it is a culture tool rather than an evidence tool.
Document version history. Google Docs and Word keep revision histories, and browser extensions can replay them. This shows typing and pasting patterns. It does not show what happened in another tab, so a student who drafted in ChatGPT and retyped it looks the same as one who wrote from scratch.
A workspace where the AI is inside. If the AI assistant the student uses is part of the same environment where they write, the record is complete: every prompt, every response, every revision, in order. This is what Cognity does. The teacher sets the task and the rules for what the AI may do, students join by link, and the teacher reads the process alongside the finished work. The AI is also allowed to refuse: if a student asks it to write the essay, it declines and coaches instead, and that refusal is in the record too.
What this changes in practice
When the process is visible, the conversation with a student changes from "I think you cheated" to "I see you asked the AI for three arguments and then wrote your own. Tell me about the second one." The student who did the work has proof. The student who didn't has a conversation about learning, not a tribunal.
That is the honest answer to how a teacher can tell. You can't, from the text. So stop trying to, and look at the work instead.
If you want to see what a process record looks like with your own class, Cognity is free for individual teachers: 100 sparks a month, two active assignments, no credit card. Set a task in about a minute and read the first record when your students submit.