judge a book by its cover以貌取人。一本书的好坏取决于它的内容，而不是它的封面，延伸为以貌取人。
judge v.断定;判断;审判。judge by (from) sth根据（由）…作出判断。

【例句】
I still believe that sentence, though not judge a book by its cover, but one's inner world in his face.
我还是相信那句话，虽说不以貌取人，但是，一个人内心世界全写在了他的脸上。

You'd better not judge him by his appearance.
你最好不要根据他的外表对他作出判断。