You must study their deliveries, their use of their bodies, their timing, and their use of audio and vocal effects.
Being a professional comedian is doing it right and good, when you don't feel like it.
If I'm crisp and economical in my delivery, have smooth transitions, movement and animation, and flights of fancy, that would get me an A.
Evaluate every performance on: stage presence, concentration, delivery, material and lessons learned. 
Don't try to give a funny opinion; give your opinion in a way that will be funny. 
Ideally, you want to be in a fifty-fifty power-sharing arrangement with the audience - both of you are there for a mutually enjoyable experience. 
When you take a pause before delivering your punch line, you will be using silence as a creative entity in itself. 
Originality is never embraced as quickly as the commonplace. 