Lighting, shadows, and reflections
One scene should have one explanation for where the light is coming from.
Find the brightest source you can see: a window, the sun, a lamp. Everything lit should agree with it, including the catchlight in an eye and the shadow under a chin. You do not need a diagram. You need a direction.
Reflections are the strict version of the same test. A mirror, a pair of sunglasses, or a wet street should contain the scene, not a generic shine. If the glasses reflect a sunset and the street is noon, believe the disagreement.
Generators are good at local prettiness. A cheek can have beautiful light that belongs to a studio while the room behind is lit like an overcast day. That mix is ordinary in composited portraits and also ordinary in synthetic ones, so it is a prompt to look further, not a verdict by itself.
When you miss one of these on the image quiz, use the zoom on the reveal and find the source you ignored.