[can16358p]

It's usually the glitchy artifacts which give the clues.

Not the face itself, bur what's around it/background/other objects etc.

Check for weird-looking "something's not right" objects/backgrounds and you'll get most of it fine.

[strken]

I got five in a row without looking for glitches or background objects, then stopped.

Look really carefully at a small area of skin. See if wrinkles, pores, hairs, and minor skin imperfections are present. See if they make sense in the context of the rest of the face.