Another test you can do while the system is at operating temperature is to touch the top radiator hose and feel how hot it is and then touch the lower radiator hose. You can do this with the engine off after driving it for a while. If the top hose is burning hot and the lower hose is totally cool, your thermostat is not working properly. If the lower hose is slightly less hot, it is working properly.

Dkr.