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]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.