How do you check a car thermostat?

Turn on the motor and leave it inactive. Look through the radiator filling collar to see if the cooling fluid is flowing. At that time, you should not run because your car did not reach the operating temperature to make the thermostat open. If you find that the cooling fluid is flowing, this means that the thermostat valve is open.Allow your car's engine to start. Do you know if the coolant flows? It shouldn't be flowing at this time because your car hasn't reached the operating temperature to cause the thermostat to open. The thermostat valve is open if you find the coolant flowing.