I find this pretty easy. The things I look for:
Surreal or screwed up backgrounds, or God help you if it tries to show a second face
If anything is obscuring the face and hasn't totally fucked up the generation, it's probably real. Fingers, hands, opaque sunglasses or microphones will never show up properly in AI-generated faces, at least for now.
Accessories that's are fucked up, sometimes the arms of glasses or earrings, headbands or hats. If it looks like it's stopping incompletely or receding into the face, it isn't real.
The edges of front teeth have a bright ridge, sometimes the teeth look "flattened" rather than properly curved around the jaw
If the face is at an angle, the eyes may be subtly different to give it away
If nothing else, the AI-generated face will never have anything soft shadows, or fully-lit from the front. Real photos will have sharp shadows cutting across the face.