It could be:
Nie szata zdobi człowieka, literally: it's not clothing that makes a person beautiful (also: nie szata zdobi człowieka, ale człowiek szatę - (...) but it's the person that adorns the clothing);
Pozory mylą - appearances are deceptive;
Nie oceniaj książki po okładce - don't judge a book by its cover.
Each of them is relatively often used in spoken language :)