There are only two types of detector certified for class C freight holds ( as fitted to 747's and the like ). The detectors are either particulate or ionising, each having advantages and disadvantages.
Particulate detectors rely on detecting the sooty particles produced by combustion, which means that they won't detect anything until the fire is actually taking hold. However, they also don't trigger fire warnings if you're carrying bananas like the ionising ones seem to do for some reason.
Ionising detectors don't need the fire to be established before they trigger a warning, but as I said above they are prone to false alarms.
You pays your money and takes your choice.
An American company ( nameless to protect the guilty) tried to market a wireless conversion for the 737's to go from D to C class holds. Their system used infra red detection. Great bit of kit - sweaty baggage loaders set it off, and on one occasion a genuine fire was raging and the wireless system triggered the alarm in the aircraft that was on stand beside it, without a peep from the cockpit module on the one that was blazing away.
Hope this helps.