To fully test something you need to experience it yourself.
The original phrase was “the proof of the pudding is in the eating” and meant that one had to taste food in order to know if it was good. The newer abbreviated phrase, which doesn’t make sense if read literally, means the same thing, that your creation is either good or bad and the end result will decide which.
When originally coined (in the 1400s, according to the OED), the phrase did not mean a creamy dessert. The OED describes the medieval pudding as “the stomach or one of the entrails of a pig, sheep, or other animal, stuffed with a mixture of minced meat, suet, oatmeal, seasoning, etc., and boiled.” Anyone who celebrates Burns Night will recognize this as a fair description of a haggis — “the great chieftain o’ the pudding-race,” as Burns called it in the poem Address to a Haggis, 1786.
Medieval people, faced with this mixture which could easily be contaminated, might have thought a taste test a wise choice. The situation is the same today. Food may have been made from fresh ingredients and look delicious, but you can really only judge by putting it in your mouth. The actual taste is the only true criterion of success.
“Proof” in the phrase is an old use of the word in the sense of “test.” The more common meaning of “proof” today is “the evidence that demonstrates a truth.” The verb form meaning “to test” is less often used now, though it does survive. For example, when bakers “prove” yeast they are letting it stand in warm water for a time, to determine whether it is active. When we read that a piece of armor is “bullet-proof” that means it’s been “bullet-tested.”
The earliest printed example of the proverb is in William Camden’s Remaines of a Greater Worke Concerning Britaine, 1605:
“All the proof of a pudding is in the eating.”
The shorter form, “the proof is in the pudding,” now increasingly common, dates back to the 1920s and came into common use in the US in the 1950s.
Another example of an old use of “proof” to mean “test” is “printer’s proof,” a preliminary “test” copy of a book printed to check for errors, before commencing a large print run.
Which is essentially what I’m doing on this intermittently sunny Sunday; checking the final manuscript of Charger the God (the third book in The Charger Chronicles series) for errors, before it’s formatted for web and print. Will the book be a success? I guess the proof will be in the reading!