>>4459050
First I get an instant gut feeling that something is wrong, like how you can tell a skinwalker isn't Human at first glance, even if I can't specifically tell what's wrong.
Secondly I then take a good long gander at the image until I find something fucky that no normal Human would make, eyes, fingers, hair, eyelids, small details stand out the most.
99 times out of 100 I'm correct in my initial assessment.