人不可貌相

jan4 bat1 ho2 maau6 soeng3

you can't judge a person by appearance (idiom) · you can't judge a book by its cover · 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]{hoi2 seoi2 bat1 ho2 dau2 loeng6}