All the Proofe of a Pudding, Is in the Eating

William Camden, Remaines of a Greater Worke, 1605

Meaning: you can only judge something after you have experienced it.

Origin: the earliest version was a medieval proverb, quoted in the poem Kyng Alisaunder (transcribed by Henry Weber, 1810): “Hit is y-writein, Every thyng, Himseolf sheweth in tastyng.”