Some are very easy to spot, like a shadow of a character, that's missing a limb on the shadow, or has different placement or pose. Illumination or parallel surfaces where they vary in shadowing without a reason, is also a dead giveaway. But the mist damning evidence is having one scene, then a slightly different scene in a reflection.

There are reasons for human authors to do any of these on purpose, but unless that purpose is part of the work, they're most likely AI mistakes.

Of course it's kind of funny how there is already a large overlap between the best AI art, and the most senseless "modern art".