Steve,
The Classic range of B-747s had individual engine nacelle temperature indicators which were in addition to the actual fire warning system. The follow up procedure following a fire warning and discharging one or both fire extinguishers as applicable was to check the Nacelle Temperature Indicator.
A reading above 7 units was a good indication that the engine fire had not been extinguished.
There was also a "Fault Light", which indicated a failure of one or both fire detection loops. A fault in both loops was also treated as a fire.
Does that help? Cheers.