If someone gave AI good enough criteria and AI made a post which should look like written by human we probably can't find out if it is not written by human. But usually those ones who've been caught on highly likely using AI for posting are not so thorough, the texts they post look unnatural, written in common words and having not much sense. Of course some real users write text the same way by themselves... but should we really care of the texts which have low quality and look like made by AI?