This is an expression that has been around for several centuries. When you tell someone that the `proof of the pudding is in the eating’, what you mean is that in order to determine the value of something, one needs to test or try it. You are telling the individual not to pass judgment on something without examining the facts or evidence. You will come to know if the pudding is good or bad, only after tasting it. The expression is often reduced to `the proof of the pudding’, and it has the same meaning as `don’t judge a book by its cover’.
On paper, your suggestions look great. But you know what they say, the proof of the pudding is in the eating.
Source: “The Hindu” – Know Your English Column – Feb 26, 2008