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.
I wonder what causes it.
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.