The thermostat is supposed to open at 180F. To test it according to the manual, you have to remove it and heat it in a pot along with a thermometer. The functional test is whether the temperature gauge on the bike reaches three bars. All the failures I've heard of involve the thermostat failing in the open position. You'll only notice a failed thermostat in cold weather (below 50F or so.) The downside of running with a failed thermostat is a loss of fuel mileage, as the engine never reaches operating temperature. During warmer months, you won't notice a failed thermostat, as the ambient temperature will cause the engine to reach operating temperature.