A good comic knows they are good. The problem is, a bad comic thinks they are good. The challenge for any comic, good or bad, is to let the material speak for itself. If it's good, an audience will let you know. If it's not, well, same thing. If you get consistent laughs, applause breaks from an attentive audience, then guess what? You did good. If you had to do half your set over chatter, with sporadic laughter, then I hate to break it to you, but that was not a set to send to Letterman.
Comics of all levels do need constant reinforcement. This is true. It can be accomplished with listening to the laughs, internal mental encouragement and genuine confidence in ones self.
Enjoy what you do as a comic. Work hard at it. Practice often and love the process. And remember, when you "kill" "crush" "destroy" and "murder" a set, give yourself credit, but keep it to yourself, or you might sound like a jack hole.