The original phrase is "The proof of the pudding is in the eating!" Which means you have to eat the pudding to know what's inside of it.
The modern version of "The proof is in the pudding." Implies that there is a lot of evidence that I will not go through at this moment and you should take my word for it, or you could go through all of the evidence yourself.
Homeowner: I don't believe it really takes 100 trees to build a house?
Carpenter: The lumber for the framing of the house requires all the boards to be the same length; the proof is in the pudding.
1. Pertaining to something that is gay and weak, being more than just gay or weak. (And not gay in the sense of sexual orientation; gay in the way that gay people use to describe something that sucks).