An image forming apparatus, detection method, and image forming system are provided. The apparatus includes a communication interface, a detector, and a notification interface. The communication interface communicates with a wireless tag at a connection position where a sheet conveyance path is formed between the apparatus and a retrofit device equipped with the wireless tag. The detector detects a connection state between the retrofit device and the apparatus based on a communication result of the communication interface. The notification interface provides a notification based on the connection state detected by the detector. The detection method includes detecting a connection state between the retrofit device and the apparatus based on a communication result of the communication interface, and providing a notification based on the connection state detected in the detection. The image forming system includes an apparatus and a retrofit device provided with a wireless tag.