“The proof is in the pudding” means that the outcome is uncertain until the task is completed.
The expression began as, “The proof of the pudding is in the eating.”
Pudding wasn’t always exclusively a dessert.
When the expression was coined, a pudding was any food presented in a solid mass and was often a main course, such as Yorkshire pudding.
Popularized in 1605 by Cervantes in Don Quixote the saying has been traced back to 1300.
Today, “the proof is in the pudding” means that you can’t tell the value of something simply by its appearance.