It just depends. I usually trust my instincts - my instincts tell me if someone is telling me to make me feel better or if they are completely sincere. If someone is sincere, I generally melt like an ice cream cone. Very messy!
If they're just being nice, I usually shrug it off and try to change the subject.
As for *giving* compliments - I try to be very open about my positive feelings for others - whether it's how they look or something they did. People around me know I'm an honest person, and know I would rather stay quiet than say anything I don't mean - so I *think* it's generally appreciated when I point out some awesomeness. I especially like randomly complimenting strangers when they're wearing something awesome or their hair is really beautiful or something like that - because they will go the rest of the day
knowing that they look good, because I have no other reason to tell them that. I feel it brightens a person's day.