Don't judge a book by its cover
Warns against making judgements based solely on outward appearance. Things and people can be very different from what they first seem.
It looks like a modest place but the food is incredible. Don't judge a book by its cover.
—I didn't like how he was dressed. —Don't judge a book by its cover.
Don't judge a book by its cover. You'll see when you get to know him better.