you can't judge a book by its cover （谚）人不可貌相，海水不可斗量

【例句】
It's just like they always say, you can't judge a book by its cover.
正如我们所说的，人不可貌相，海水不可斗量。

Mary may look innocent and naive but she's very mature and knowledgeable, so it shows that you can't judge a book by its cover.
玛丽看上去天真无邪，但是她十分成熟，而且很有学问，这说明不能以貌取人。