judge a book by its cover, one can't

you can't judge a book by its cover

proverb 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.
See also: book, by, cover, judge
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

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]
See also: book, by, judge, one
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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