I don't have an expert knowledge of it, but as far as I know, there are a few things they tend to look for: the eyes, the lips, the general outline of a head, and large areas of the image which fall in particular colour ranges (white skin is much easier than dark here - hence the 'racism'). There are others, but I'm trying to recall a casual conversation with a postdoc two or three years ago. Maybe Martin has some familiarity with the problem?