The one thing that we can prove with reasonable ease is that the time it took to write was compatible with the author being a human, and that the writing signature is that of a human. You can still fake it, but it is much more annoying and it will take you at least as long as it would take a human.

I wrote Possibly Made by a Human just for fun to do this, after I failed the Turing test in a comment here.

Actually made by a human: https://possiblymadebyahuman.com/H1LfVy72BZ