This might help...
http://www.addictionca.com/signs-of-cocaine-us e.htm
Check their nose out and eyes (often bloodshot).
Being very agitated, almost hyperactive and overly confident is common in someone who's taken it (often highly sexed as well).
Doing thing they normally wouldn't have the confidence to do.
Partying to the very early hours (seemingly like they have limitless energy on nights out).
Severe headaches is something I've noticed in someone I knew who used to take it a lot.