It used to be easy by comparing reflection on eyes - one was completely different than the other. Or teeth line. I can see they've improved it a lot. I can't tell the difference now.
Just about all of these images have a characteristic "swirl" somewhere. Weird ears and eartings, impossible hair textures, confused "hats" and nightmarish hands/other faces in the image are also clear tells, but that little swirl is usually there even in otherwise "perfect" images.
It's hard to learn high frequency gradients. It's easy to learn a single edge, but not a large pile of overlapping gradients like the ends of hair.
I don't know why it's so common to have an arbitrary nonsense "sticker" on a face though, I wonder if it's because input photos were trunacted at edges and the learner tried to model the edge as a facial feature instead of pruning it's knowledge to a safe interior of the photo.
