the.com/proof of the pudding

means The real test of whether something works is trying it out in practice, not theorizing about it beforehand.

from Shortened from the old saying 'the proof of the pudding is in the eating' — 'proof' here meaning 'test' (as in a proving ground), not evidence after the fact. Puddings in the old sense were boiled or baked mixtures that could look fine and still turn out wrong inside, so the only honest test was to eat the thing. The saying dates back centuries in English; the clipped modern phrase 'proof is in the pudding' is a later, looser version that drops the 'eating' — which, strictly speaking, hollows out the original logic, since a pudding sitting there proves nothing until it's tasted.