know one for what one is

know (someone or something) for what (they or it) is

To identify or recognize someone or something for the type of person or thing that they truly are. "What" can be replaced with a different noun. By now I know these offers for what they really are—shameless scams. He tried to impress her with his charm and wit, but she knew him for the scoundrel that he was.
See also: know, what

know one for what one is

to recognize someone as some type of person or thing. I know you for what you are, you devil. We know him for the thief he is.
See also: know, one, what