Dic: The proof is in the pudding

Prov. You cannot be sure that you have succeeded until you have examined the result of your efforts.

• Jill: I think we've done a good job of fixing the lawn mower.
  Jane: Well, the proof is in the pudding. We haven't tried to mow the lawn with it yet.

Cf. The proof of the pudding is in the eating.

Prov. You don't know the quality of something until you have tried it or experienced it.

• Theory says that this material will produce a superior widget, but the proof of the pudding is in the eating.

.... McGraw-Hill

CF. The proof of the pudding (is in the eating)

the way to judge something is by its results

• Economic growth is the proof of the pudding for the president's tax and budget plans.

.... Cambridge Dictionary of American Idioms

... http://idioms.thefreedictionary.com/proof