For me, the best indicator if a post is AI-generated is if it is written in flawless English. Even the best posters who are native English speakers don't necessarily write every post using the precision in punctuation, spelling & grammar executed by ChatGPT. If a user's native language isn't English (as can be evidenced by their posting history), and they suddenly start posting in flawless English, it means one of two things:
1. The text is plagiarized, or
2. The text is AI-generated.
It is not fair to assume that someone who writes in flawless English is necessarily plagiarizing or using AI-generated text, and I disagree with this assumption because I believe there could be a third possibility.
In my opinion, @joker_josue makes a valid point. It is difficult to differentiate between content generated by ChatGPT and text that has been translated from a non-English native language to English or edited using ChatGPT and similar tools. I do not think there is a clear way to distinguish between the two, and I suspect that most AI Content Detectors would classify the text as AI-generated, even if it is not the case.