"The proof is in the pudding'' is actually a shortened version of a very old proverb, "the proof of the pudding is in the eating.'' It means that the real worth or success or effectiveness of something can only be determined by putting it to the test, appearances and promises aside - just as the best test of a pudding is to eat it.
Via:
http://www.jewishworldreview.com/070..._men070804.asp