The proof is in the pudding.
Pudding, as opaque and thick as it is, could potentially have a lot of things lurking in it. The proof is in the orange juice, the proof is in the tuna sandwich; these are less likely scenarios. Pudding could conceivably contain proof, victory, old tennis shoes, and my long gone stolen laptop.
‘The proof is in the pudding’ is a decomposed and reassembled version of the original idiom, ‘The proof of the pudding is in the eating’. ‘Proof’ having been in its now mostly antiquated form (except for in that god-forsaken high school math class) meaning ‘test’. In other words, you have to give something a try before knowing its value.