Light is applied by a laser diode 121 onto a photosensitive roll 110 formed with a developed toner image 161, the surface potential of the photosensitive roll 110 to which the light is applied is measured with a surface potential sensor 122, and the toner amount of the developed toner image 161 on the photosensitive roll 110 is derived based on the measured surface potential. Generally, a sensor assembly contains a plurality of current transformers for measuring power wires from a main power supply. Fact checking results are able to be validated by re-fact checking the fact check results.