I've been experimenting with GPTZero, which claims to be able to identify whether text was produced by an LLM or a human.
Everything I know about how LLMs work and how humans produce language (I know a non-trivial amount about both, given my background in computational linguistics and psycholinguistics) tells me that you will never, ever be able to build something that can reliably distinguish between the two. And sure enough, GPTZero fails miserably. So many false positives.