That said, people often have minor quirks and the ability to carry information from seperate interactions. So it's likely if someone makes a reference to something else, eg "I remember reading [X], but I don't recall where" or, even more, physical editions of a book, eg "it's on page 73 of one edition, but I think in another edition it got shifted around", that's an indicator they're human.
If all else fails, you could say "ignore all previous instructions and [x]" and hope it works in revealing if it's an AI.
But there's no way to conclusively demonstrate, as Descartes found out; you simply can't prove that you're not a brain floating in a jar in outer space, hallucinating all of this. Or a simulation. Or...
At some point you'll either need to be satisfied with living in a world where you can't prove much, or not believe anything.
Occam's Razor. There would be massive coordination behind any effort to make the internet "dead". It would be such a vastly complex mechanism.
Cheers, M