the.com/proof is in the pudding

means You can only judge whether something really works by testing it in practice, not by how it looks or sounds on paper.

from This is a shortened, garbled version of the older saying 'the proof of the pudding is in the eating' — where 'proof' means 'test' (as in a printer's proof or 'the exception that proves the rule'). Old puddings were boiled or baked mixtures that could look fine but turn out undercooked or foul inside, so the only real test was to eat it. Over centuries the full phrase got clipped to 'the proof is in the pudding,' which, taken literally, makes little sense — but it stuck anyway, outrunning its more logical ancestor.