The most pressing sign is a lack of remorse. Empathy is not necessarily impossible -- just highly selective, to the point where it's arguable that empathy is only possible if the subject is also a narcissist and thus only relates to people much like himself. Habits of deception and manipulation are common, performed daily. Maybe even compulsively. The most interesting quality -- beyond the cruelty and tendency to dehumanize others, I mean -- is the flagrant disregard for social norms.

Fascinating, isn't it?