1. An image forming apparatus comprising: an image carrier configured to support an electrostatic latent image; a charger configured to charge the image carrier to an electrical potential according to an applied voltage; an exposure unit configured to form the electrostatic latent image by exposing the image carrier; a developing unit configured to develop the electrostatic latent image into a developer image by adhering a developer to the image carrier; a belt; a transferring unit configured to transfer the developer image from the image carrier to the belt; a detection unit configured to detect a density of a developer adherent to the belt; and a controller configured to: determine a deteriorated state of the image carrier by charging a portion of the image carrier, subsequently subject the portion of the image carrier to a development using the developing unit without subjecting the portion of the image carrier to exposure by the exposure unit, in a condition where a potential difference between the charger and the developing unit is set to be smaller than that at a time of image formation, transfer the developer on the portion of the image carrier to the belt by the transferring unit, and detect, with the detection unit, the density of the developer transferred to the belt.