1. hands
2. eyes
3. look at edges and repetitive patterns, for... it's kinda hard to describe what, exactly, but a certain kind of weirdness (e.g. in the social preview image of the robot painting, check out the grid thing on the right)
4. look below the semantic / representation level, at the medium: Are there brush strokes? Do they make sense?

Basically the same as Ben but adding:
- incoherent objects, especially in the background (eg. The grapes in the still life aren't organized into a bunch, there's a temple where some of the columns do a sort of Escher thing)
- re: edges and repetition: I think the specific signal here is often a sort of organic "blobbiness" or "spider-webbing" that doesn't match the expected style or subject matter. So like: varying line weights in a cartoon, windows that aren't actually square, embroidered detail on a tunic that doesn't look like anything, hair that doesn't attach to a person's head

I think it's easier to positively identify something as generated based on the presence of artefacts than it is to be sure something is human. The only one that I was >90% sure was human was the big spaceship.