You can't judge a book by its cover

ɡʌmmosɯpmɐn poqo phɐndɐnhɐdʑimɐsejo dʒiːəʊtməʊsʌpmən bəʊɡəʊ pandənhadʒi meɪsjəʊ

In Korean 겉모습만 보고 판단하지 마세요 (Geotmoseupman bogo pandan-haji maseyo)

This proverb is used in Korean culture to advise against judging based on appearances.