What does it mean if a guy asks if he's a poser?

A poser is a person who pretends to be someone he is not. He is putting on a pose or posing as someone he is not. Ex., he works in a gas station, but he shows up at a bar in an Armani suit, wearing a (fake) Rolex. Closely related to affectation, where on affects certain mannerisms to give credence to a pose. Ex., he affects a Northeast prep-school accent to add to the illusion of wealth.