You can only say something is a success after it has been tried out or used.

Example

"I know you didn't think it was a very good product, but just look at the fantastic sales figures. That's the proof of the pudding."

Alternative forms

• the proof is in the pudding

Etymology

Etymology This proverb dates back at least to the 14th century as "Jt is ywrite that euery thing Hymself sheweth in the tastyng", and William Camden stated it in 1605 in Remaines of a Greater Worke, Concerning Britaine as "All the proofe of a pudding, is in the eating", per Rogers' Dictionary of Cliche and the Oxford Dictionary of Quotations.

A 1682 translation of Nicolas Boileau-Despréaux Le Lutrin (written between 1672 and 1674) renders it "The proof of th' pudding's seen i' the eating."

The current phrasing is generally attributed to the 1701 translation by Peter Anthony Motteux of a proverb Miguel de Cervantes used in Don Quixote (1615), al freír de los huevos lo verá (“you will see it when you fry the eggs”).

The shorter form the proof is in the pudding, which is found in an 1867 issue of the British Farmer's Magazine, and came into common use in the United States in the 1950s, is becoming increasingly common, despite missing the point of the original meaning.

Sources

• Wiktionary