Having a car that warms quickly is particularly useful in cold weather when you want to use your car heater as soon as plossible. The thermostat is in a small housing positioned where the top radiator hose connects to the engine. The best time to check or change a thermostat is when you are servicing your radiator because you need to drain the cooling system first. Hence, a thermostat change is a good time to flush and clean your cooling system, check all radiator hoses and the radiator pressure cap. Be sure you have the correct cap on your radiator and it is not rusted.
Thermostats are small, inexpensive items.
You can test them by heating in water and checking their operating temperature with a thermometer. However it really is quicker and ultimately more reliable to simply fit a new one. Put the car on level ground. Drain the cooling system and unscrew the thermostat housing and connecting water hose. Look at how the thermostat is positioned and remember to put it back the same way.