Other people might point to more specific tells, but instead I'll reference https://zanlib.dev/blog/reliable-signals-of-honest-intent/, which says that you can tell mainly because of the subconscious uncanny valley effect, and then you start noticing the tells afterwards.

Here, there's a handful of specific phrases or patterns, but mostly it's just that the writing feels very AI-written (or at least AI-edited). It's all just slightly too perfect, like someone's trying to write the perfect LinkedIn post but are slightly too good at it? It's purely gut feeling, but I don't think that means that it's wrong (although equally it doesn't mean that it's proven beyond reasonable doubt either, so I'm not going to start any witch hunts about it).