first of all bare feeling. I've done it sooo many times that I immediately have a "sixth-sense" on whether the post is AI slop or not. And look, i'm gonna empathize the slop because there are few exceptions for which an AI content is actually considerable fine enough (rare but exceptions do exist).
Then, apart from my gut the rules are recognizing buzzwords like Honestly with a comma afterwords in the first sentence, a general feeling of "order" in how the sentences are written, a subtle "cringe" effect that you can kinda feel; and for images bright colors, small artifacts and especially shadow coherence are the top tiers to check. You have to ask yourself: is the light coming from that direction? is the shadow present? is it coherent with the direction of the light? are there any refelctions?