Не суди об арбузе по корке, а о человеке - по платью

Do not form an opinion about a person or thing based on appearance alone. See Внешность обманчива (B), Не всяк монах, на ком клобук (H)

Cf: Don't judge a horse by its harness (Br.). Don't judge a man by the coat he wears (Am.). Judge not according to appearances (Am.). Never judge from appearances (Am., Br.). You cannot know the wine by the barrel (Br.). You can't judge a book by its cover (binding) (Am.). You can't judge a horse by its harness, nor people by their clothing (Am.). You can't tell how far a frog will jump or a horse will run by the color of his hide (Am.)