the system used is sometimes called "THE LINDBERGH LOOP" and is substantially a thermocouple. Heat impacts bi metal creating a voltage and triggers an alarm.
so there you go.
with cameras being so small now, I would think a camera in the nacelle, at least the area protected by the fire bottles, would be a nice thing. also, for engine surges/compressor stalls, cameras aimed at engine from fuselage or outboard wing.