What does this mean?

One should not accept people at face value?

The same as "don't judge a book by its cover" -- http://www.google.com/#hl=en&q=idiom+don%27t+judge+a+book+by+its+cover&aq=0&aqi=g1&aql=&oq=idiom%3A+don%27t+judge&gs_rfai=&pbx=1&fp=d331bd8e2d0de10c