We say…

Don’t judge a book by its cover. It means you shouldn’t judge the value of something or someone based on their appearance.

Examples:

Maria is only fifteen years old, but she is very mature and knowledgeable, so it shows you can’t judge a book by its cover.

When we arrived, the hotel looked amazing – but once we got inside the rooms were dirty and damp. Well it just goes to show, don’t always judge a book by its cover!