Barnaby Rudge

judge a book by its cover

To make judgments about things or people by considering external appearances alone.

judge, jury and executioner

Someone with the roles of judge, jury and executioner; someone with full power to judge and punish others unilaterally.

tell it to the judge

I do not believe what you said.

you can't judge a book by its cover

It is not possible to make reliable judgments about things or people by considering external appearances alone.