I find it really easy to tell the difference between a human being polite, neat and well-spoken, and an AI being the same (but soulless). I don't know if I could put it into words though, there's just something about AI that lacks subjectivity? A human would phrase something in a certain way and stick with it, because that's the way they experience it, while the AI takes a phrasing at random, only caring about gaining lexical and grammatical points.

I also think humans overestimate their ability to write clearly and correctly. There's always some noise in there, even if they're going full corpo-speak. Unless it's written-by-committee meaningless corpo, but then I don't even read it beyond the first sentence. It's very obvious when someone has tried to strip all meaning from a sentence and the result is not far from AI.