The Lightning's fire warning system was pretty good, and generally would set the Standard Warning Panel going before any external signs of fire became visible, even to a formating aircraft. The system was activated when the Instrument Master Switch was on, and its serviceability was tested via a button on the SWP during pre-flight checks.
Which is why I am puzzled about reports and photographic evidence of external smoke and fire when the pilot's radio transmissions were apparently about hydraulic problems.