What are some colloquial ways to convey the idea of
"You can’t judge a book by its cover"
meaning one should not judge something or someone by how it, he or she looks - it's what's on the inside or the content that matters most.
But you must never judge a book by its cover!
I would say
Mais (il ne) faut jamais juger un livre par sa couverture !
Linguee affirme that the expression française correspond à l'expression anglaise. But that majority of sources are canadiennes. Also, I think that judge in English is more frequently used that the verb juger in French. Lastly, I don't recall hearing a native to use this expression.