Without a fire warning system I'd imagine you're limited to external observation by others prior to the donk dying.
As I started my PPL a few years back we had a double belt failure on a 22 resulting in the oil cooler pipes getting sliced. The only indication of the resulting fire was the reflection of the flames on the apron concrete. According to the tower, there wasn't much in the way of smoke.
The AFS chappies (wonderful guys) tell me it's rare to get much smoke due to the big fan on the roof rack...fair point. So short of a heat sensing system monitoring the interior compartments, the most likely indicator I should imagine would be low oil pressure followed by overtemps and then the big quiet bit.
But I'm no expert... so as someone who only flies a 22 now and then (also no fire warning) I'd be interested in the knowledge of others, myself.