Don't judge a book by its cover.
 → 겉만 보고 판단하지마.

 ※ 책표지만 보지말고 속내용을 보라는 말이며,
  즉 사람의 외관으로 판단하지말라는뜻.

[예문]
A:Have you met our new teacher? He looks very week and short.
   새로 부임한 선생님 봤어? 키도 작고 되게 약해보이더라.

B: Don't judge a book by its cover. He is a martial arts expert.
   겉만 보고 판단하지마.무술고단자래.

A: Oh really? I didn't know that.
   정말이야? 몰랐는걸?