Meaning of 是骡子是马,牵出来遛遛

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shì luó zi shì , qiān chū lai liù liu (Trad.: 是騾子是馬,牽出來遛遛)
lit. to see whether it's a mule or a horse, take it out for a walk (idiom); fig. the proof of the pudding is in the eating; to show what one is made of