How do you recognize a sociopath based on appearance or behavior?

Appearance - You can't. The only outwards sign can be a piercing stare or a sense of evil eminating from them.

Behaviour- You can't unless they let their guard down. The main behaviour is their manipulation toward their own aims.

More on the signals of a sociopath:

http://wiki.answers.com/Q/How_do_you_know_if_someone_is_a_sociopath