I've seen a fair number of cases where someone swears up and down not to be using AI to generate responses, but there's no good reason to believe it (except perhaps specifically for the messages where that claim is made).

This includes times that someone basically disappeared from e.g. Stack Overflow at some point before the release of ChatGPT, having written a bunch of posts that barely demonstrate functional literacy or comprehension of English; and then came back afterward posting long messages with impeccable grammar and spelling in textbook "LLM house style".

In one study, GPT-4.5 was judged to be human 73% of the time, which means that the actual human was judged to be human only 27% of the time. More human than human, as Tyrell would say.

Edit: folks, the standard Turing test involves a computer and a human, and then a judge communicating with both and giving a verdict about which one is the human. The percentages for the two entities being judged will add up to exactly 100%. That's how this test was conducted. Please don't assume I'm a moron.