是骡子是马，牵出来遛遛
shì​luó​zi​shì​mǎ​qiān​chū​lai​liù​liu

the proof is in the pudding

是骡子是马，牵出来遛遛 是騾子是馬，牽出來遛遛 shì​luó​zi​shì​mǎ​qiān​chū​lai​liù​liu
the proof is in the pudding
literally meaning "to see whether it is a mule or a horse, take it out for a walk"; used to mean that a person's true ability or the quality of a thing can only be judged through a practical test