I've done the both. Crack, not as much as cocaine. With Coke, a major sign is a lot of sniffling. Obvious reasons. They really don't pick as much as someone on crack or meth. Look for them to constantly clear their throats, lick their gums, move their mouths a lot, become extremely talkative and very easily amused.
Crack, I guess really the main sign is picking. Not just at themselves, but also at the carpet, furniture, stuff like that.