An example method of operating an image forming apparatus includes measuring a toner concentration supplied to a developing apparatus and increasing a driving speed of the developing apparatus based on the toner concentration not reaching a target toner concentration. The toner concentration is measured in a state where the driving speed of the developing apparatus is increased, and, based on the toner concentration not reaching the target toner concentration based on the driving speed of the developing apparatus being increased, an operating state of the image forming apparatus is determined based on a remaining amount of toner.