Image forming apparatus determining states of members, image forming method, and image forming system

A sensor unit attachable to an image forming apparatus, comprising:

• a transmitting member configured to transmit a first sonic wave;
• a receiving member configured to receive the first sonic wave that has been transmitted from the transmitting member and has passed through a recording material, and a second sonic wave that is generated when the image forming apparatus operates for forming an image on the recording material, wherein the receiving member is configured to receive the first sonic wave and the second sonic wave in a state in which the sensor unit is attached to the image forming apparatus;
• and
• a control unit configured to output a first signal, in a first mode, according to the first sonic wave that is received by the receiving member, and to output a second signal, in a second mode, according to the second sonic wave that is received by the receiving member.