You can’t judge a book by its cover

Meaning: we can’t judge someone’s character only by their appearance.​

Example sentences:​

The man wears simple clothes and drives a cheap car, but I’ve heard that he’s one of the wealthiest men in the country. You can’t judge a book by its cover.

The building looks shabby but inside it looks straight out of a luxury magazine. You can’t judge a book by its cover.