know your tailor

To know the person that is giving some informations about you, creating your image in front of other people. It can be both, positive and negative statement.

A - Somebody is telling some image-building things about you.
B - Yeah, I know my tailor, it's Clair from the first floor.

or

A - Who can tell those smearing things about me?
B - You have to know your tailor before you accuse somebody.