The thermostat on a trials bike is a very simple device. Unlike a car where the thermostat controls the circulation of water, the one on your bike simply controls the supply of electricity to the cooling fan.
To test it make up a simple testing device from a couple of wires and a battery and bulb to make a circuit. Disconnect the wires to the thermostat and attach the wires to your testing device. Now run the engine to get it up to working temperature and watch to see if your bulb lights. If it does then th thermostat is fine if not its knackered !
Alternatively just buy a new one and keep the old one for a spare if it doesn't cure your problem