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 phrase may also be used in the past and future tenses: the proof will be/was in the pudding.
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.
It was a particularly apt phrase because pudding did not mean a creamy dessert, it was the term for a kind of mixture of meat and seasonings stuffed into a casing like sausage. And with meat in the very early 1600s, when the first example of the idiom appears in print, or in the 1400s, when the OED states the phrase originated, it was very easily contaminated. One would not know if it was going to be good or bad until one ate it.
“The most important thing for our organization is for him to develop. … The proof is in the pudding at this point.” [The Washington Post]
“We think we’ve been productive over the last two weeks but the proof is in the pudding on Sunday.” [Western Morning News]
But Boyd has worked hard to turn his life around and the proof was in the pudding when Wayne Bennett’s newest Broncos signing was unveiled at Red Hill on Monday. [Stuff NZ]
“I guess the proof will be in the pudding when we see how we react to it all, but we’re excited about what happened.” [Los Angeles Times]