The saying "never judge a book by its cover" or "don't judge a book by its cover" means that you shouldn't judge someone or something based only on what you see on the outside or only on what you perceive without knowing the full situation.
The saying "never judge a book by its cover" or "don't judge a book by its cover" means that you shouldn't judge someone or something based only on what you see on the outside or only on what you perceive without knowing the full situation.