proof of the pudding

The ultimate evidence attesting the true nature of something: The proof of the pudding is in the election results, not the polling.

[From the proverb The proof of the pudding is in the eating.]

Proof means ‘test’ rather than the more normal ‘verification, proving to be true’.

Jt is ywrite that euery thing Hymself sheweth in the tastyng.
[c 1300 King Alisaunder (EETS) l. 4038]

All the proofe of a pudding, is in the eating.
[1623 W. Camden Remains concerning Britain (ed. 3) 266]

As they say at the winding up, or the proof of the pudding is in the eating.
[1666 G. Torriano Italian Proverbs 100 (note)]

The Proof of the Pudden is in the Eating.
[1738 Swift Polite Conversation ii. 132]

With respect to the scheme‥ I've known soldiers adopt a worse stratagem. ‥There's a proverb however, I've always thought clever‥The proof of the Pudding is found in the eating.
[1842 R. H. Barham Ingoldsby Legends 2nd Ser. 25]

Let us‥look at the thing more widely. The proof of the pudding is in the eating.
[1924 J. Galsworthy White Monkey iii. xii.]

Lindzen for one argues that if the models get the detail wrong, they will get the big picture wrong, too. But modellers say the proof of the pudding is in the eating.
[1997 New Scientist 19 July 41]

Related to: appearance; reality and illusion

Bibliography of major proverb collections and works cited from modern editions is available here.

Results are what count, as in Let's see if this ad actually helps sales--the proof of the pudding, you know. The full expression of this proverb, dating from about 1600, is The proof of the pudding is in the eating, but it has become so well known that it is often abbreviated.