you can't judge a book by its cover
You cannot determine or should not judge the true qualities or characteristics of someone or something just by assessing superficial details alone. He seemed like a bit of a jerk to me, but I guess you can't judge a book by its cover.
judge a book by its cover, one can't
One can't rely on outward appearances to know what something or someone is really like. For example, He seems very quiet, but you can't judge a book by its cover. [First half of 1900s]