proof of the pudding, the - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.