Bring a pot of water to a boil. Once its boiling, take the pot off the stove and drop the thermostat in. The thermostat will close. Stick a meat therometer in the water. Now watch the thermometer temp go down and listen for the click. When it clicks it works. The thermo should open around 170 or 180 degrees.

Or you can just go by a 6 dollar thermostat and change it.