"You can't judge a book by its cover."
Significa que no podemos dar una opinión de algo o alguien sin conocerlo a profundidad primero. No nos podemos dejar llevar por las apariencias.//
It means that we cannot give an opinion of something or someone without knowing it in depth first. We cannot get carried away by appearances.
The Spanish phrase equivalent to this phrase is:
Caras vemos, corazones no sabemos.
