Maybe you guys flying the small stuff are still using hand signals, but all heavy jets start in communication with an engineer/ operative on the ground intercom.One company I worked for did not allow starts on hand signals only and had a call ' flames in No... engine' from the ground crew to alert the flight crew to a jetpipe fire.This is an non-hazardous event if it is dealt with correctly: fuel off, continue to motor the engine until the flames go,starter off. If it is left to burn ( incorrect drill or no way to motor the engine) there is a threat to the engine/aircraft structure.The EGT sensors, readable in the cockpit, are at the rear of the turbine stages of the engine and only sense the peak gas temperature within the engine. There are no sensors in the engine exhaust duct and the flight crew have no indication whatsoever of a jet/tailpipe fire- any warning will come from ground crew or ATC observing the event. I once had an engine go sub-idle taxiing in which caused a jetpipe fire. It was ATC who saw it first.