人不可貌相

rén bù kě mào xiàngrenbukemaoxiang

1. you can't judge a person by appearance (idiom)
2. you can't judge a book by its cover
3. often in combination 人不可貌相，海水不可斗量[ren2 bu4 ke3 mao4 xiang4 , hai3 shui3 bu4 ke3 dou3 liang2]

Study it in Yanamino