A similar expression "Don't judge a book by the cover"
Which may elude to superficial snap judgement as well as the possibility of an image misrepresenting the content.
Some other attempts to define it
http://english.stackexchange.com/questio...
It's typically more about not believing deceptive scam artists making some false claim, which may be the value, ownership, or functionality of something.
It's a little confusing when Face Value is defined as:
"The value printed or depicted on a coin, banknote, postage stamp, ticket, etc., especially when less than the actual or intrinsic value"
When paper money has been printed by someone with no authority it may not be redeemable for the marked value.
It is an expression, and it means you question everything, and are a little distrustful that people do not mean exactly what they say.
It means you do not tend to assume a situation is what it appears to be or that a person is telling you the whole truth. You are a skeptic.
