If something like this was implemented, the benefit of the doubt would have to be given in ambiguous cases, but I don't think it's that hard to tell most of the time.

The latest codemaxxed models all tend to write in very distinct, instantly recognizable ways unless carefully instructed otherwise (honestly a good thing if you want to avoid wasting time reading AI text). A great example is this submission that's currently #1 on the front page (which is also just a thinly veiled advertisement): https://news.ycombinator.com/item?id=48884853