This idiom expresses the idea that in order to know something’s true value or to really know what something is like, you have to try it out first. Sometimes, the corrupted phrase “The proof is in the pudding” is used. You can use either, though the title one is recommended.
Yolanda: Oooh, you made pudding!
Angelica: Yeah, last night. It’s my first one ever.
Yolanda: Have you tried it? Is it good?
Angelica: Well, the proof of the pudding is in the tasting! Here’s a spoon. Let’s try it together!