The idiom is usually stated the proof is in the pudding and means that the end result is the mark of the success or failure of one’s efforts or planning.
The original phrase was the proof of the pudding is in the eating and was generally used to say that one had try food in order to know if it was good. The word proof is more synonymous with test than evidence. The pudding is good or bad and the trial of eating will decide it; rather than evidence that would prove it is in fact pudding.