the proof is in the pudding
The final results of something are the only way to judge its quality or veracity. Our analysts think that the marketing campaign will reinvigorate our sales, but the proof is in the pudding, so let's see how our figures look at the end of the year. OK, if I did everything right, the engine should work right, but the proof will be in the pudding.
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.