After 5 minutes, I got tired and started seeing some of the same pictures again. 100% right all the time. For me, the trick is to assess the background, ear shape, synthetic textile (if any), and skin conditions.
For me, the backgrounds were interesting for their _more salient_ features rather than their ambience, e.g. unexpected smears of color, textures that vaguely looked like real things at a glance (like fabric or nature) but wouldn't stand up to scrutiny. They reminded me of the typical "mistakes" that you see when playing around with image generators.
Same but I focused on their expressions. Faces with expression “we are taking a picture of me” were fakes.
But that being said, all the pictures were insanely convincing and I picked fakes only because I knew I had to pick one and not because I knew one was fake.
2 more potential things to check : digital artefacts on teeth and between hair and background
Now they know what to improve ... Next round will be more difficult ...