While you can definitely ask a friend whether Marcel Proust’s In Search of Lost Time is really worth reading, you won’t know if they’re right until you slog through all seven volumes yourself. After all, the proof is in the pudding—or the best way to determine the value of something is to experience it firsthand.
As is often the case with idioms, this one was originally meant quite literally. According to the Oxford Dictionary of Proverbs, the earliest known written reference to the phrase comes from English antiquarian William Camden’s 1623 volume Remains Concerning Britain; and similar maxims (though not necessarily pudding-specific) date as far back as the 1300s. During those centuries, pudding wasn’t the gloopy dessert that Americans eat today—it was a hodge-podge of minced meat, spices, cereal, and sometimes blood, all crammed into a sausage-like animal casing and steamed or boiled. Since preservative techniques were rudimentary and food regulatory agencies didn’t exist, there was always a chance that a meat dish could sicken or even kill you. Unfortunately, as Merriam-Webster explains, the only way to find out if it was dangerous was to dig in.
It’s really less about the proof being in the pudding and more about eating the pudding to find the proof. Which brings us to our next point: “The proof is in the pudding” is actually an abbreviated version of the full phrase, “The proof of the pudding is in the eating.” The fact that people have shortened it over the last several centuries isn’t exactly surprising—phrases often evolve in ways that affect our understanding of them. “One bad apple,” for example, is really “one bad apple spoils the whole bunch,” though people sometimes use the truncated version to mean exactly the opposite.
We don’t know exactly when “the proof is in the pudding” started eclipsing its wordier (albeit clearer) ancestor, but it’s been in our vernacular since at least the 1860s. As Grammarphobia reports, engineer Henry Dircks used it in his 1863 novel Joseph Anstey, and it appeared again in an 1867 issue of The Farmer’s Magazine. Considering the abridged adage is roughly 160 years old, you can probably use it freely without fear of being corrected.