What indicates that an APU fire has occurred during operation?

The indication of an APU fire during operation is signaled by a steady light in the APU fire pull handle. This steady light serves as a clear visual alert to the crew that a fire condition has been detected within the Auxiliary Power Unit. The design of the warning system is meant to direct attention to the appropriate action that needs to be taken, which typically involves pulling the fire handle to activate the fire suppression system.

Other potential indicators, such as a flash on the cockpit display or a flashing indicator on the fuel gauge, may suggest various types of malfunctions or alerts, but they do not specifically indicate an APU fire. A change in engine RPM is more closely related to the performance of the main engines rather than the APU status, and thus is not a reliable sign of fire in the APU. The steady light on the APU fire pull handle is the definitive signal that requires immediate action for safety.