Looking at detail in general, especially stuff that's complex like wrinkles, knots, frills and patterns around edges. AI is really bad at small details like that and keeping them stable, especially when compared with each other. Perspective can be really off too sometimes. Other stuff to look for: hair strands, the height of trees in the background, clouds, clothing trim, reflections, etc.

I heard if you use Photoshop (or something similar) you can use that to show JPG artifacting in machine-sludge images too even if they're lossless files, though I didn't fully grasp that fully.