The Phrase Finder explains that the longer version “the proof of the pudding is in the eating” makes more sense and that the often quoted “proof is in the pudding” makes no sense. It means “To fully test something you need to experience it yourself.” The Oxford Dictionary of Quotations dates it to the 14^th century. The first written example located was dated 1605 and was “All the proof of a pudding is in the eating.” It is speculated that “pudding” in the original usage was what we would call sausage today.