You can't judge a book by its cover means that before you can judge something, you need to take a deeper, closer look at it. Example: "He dresses in plain clothes and drives an old car. Who would know he is the richest man in town?" Reply: "You can't judge a book by its cover."(Goenglish.com)You need to see Susan Boyle on "Britain's Got Talent"
Watch the video here. (they don't let you embed it).