Apparently that phrase is incorrect and should be “the proof of the pudding is the eating”. You know what I say to that? BOLLOCKS!!! They’re both legit and have different meanings; the latter means ‘you can’t judge something unless you put it to the test’, as for the former ‘take a look for yourself if you’re sceptical’.
To put the pudding analogy to the test, say there’s an apple pie and you’re curious to know what’s inside, because you’re unsure it is apple, you can’t see the contents because it’s hidden within the pastry goodness. The only way to know what’s inside is to look inside the pudding.
There is a lot of technicalities surrounding that, for example; an apple pie isn’t technically classed as a pudding, the pastry layer on top is usually latticed so you can see the mushed apples and you can’t really take a look inside an apple pie unless it has been served, otherwise you’ve ruined the pie for everyone else, but you do get the idea with that. A Christmas pudding would have made a better example, with the 50p coin some people put inside, trying to choke one of your in-laws in some kind of sick, festive Russian Roulette, along with the rest of your family.