That is, how do we tell if a post was written by a human or a robot or AI? Is there a way to do it beyond any doubt?
There is always doubt, but there are plenty of checkers out there already. Are they infallible? unlikely.
However, when we go to a concert and see a band play, we know we are seeing the band, right? At some point, there will likely be similar checks for digital content. A fingerprint of some sort.
Because if we can't tell it apart with the naked eye, we risk ignoring or penalizing content that was written by real people and not by AI.
It is pretty easy to tell if you know what to look for. However, one good way is to see what kinds of comments they give in replies - that tells a lot.