Wiktionary
you can't judge a book by its cover
alt. (lb en idiomatic metaphorical) It is not possible to make reliable judgments about things or people by considering external appearances alone. prov. (lb en idiomatic metaphorical) It is not possible to make reliable judgments about things or people by considering external appearances alone.
Wikipedia
You Can't Judge a Book by Its Cover (Desperate Housewives)
“You Can't Judge a Book By Its Cover" is the 77th episode of the ABC television series, Desperate Housewives. It is the seventh episode of the show’s fourth season and originally aired on November 11, 2007, in the United States.