Two ways to check if a thermostat is working properly are the radiator coolant level test and the radiator hose test. Either can determine if the thermostat is "stuck" and not directing the right amount of coolant into a car's engine, causing the radiator to overheat.
The radiant coolant level test should be done on a flat surface, with a tire chock, block of wood or a large rock placed in front of a rear tire. Since it involves running the engine with the hand brake on, this keeps the vehicle from accidentally moving. The radiator cap is removed and the car is started and allowed to run.
The tester then stands at the front of the car, preferably off to the side, and watches the fluid level in the radiator. If the coolant level in the radiator starts to drop as the engine warms, the thermostat is fine. If the coolant level stays the same, the thermostat is most likely stuck.
The radiator hose test involves starting the engine and letting it warm up, then turning it off. The hood is opened to see how much heat is radiating from the engine, signaling a possible overheat. If there is a lot of heat, the car is allowed to cool and this step is repeated.
After the second heat up, the radiator hoses are checked for heat. One hose goes into the top of the radiator, the other connects to the bottom. The tester quickly touches both hoses to see if they are roughly the same temperature. The lower radiator hose should be warmer than the top, but not by much. If one hose is extremely warm, and the other significantly cooler, the thermostat is probably stuck.