Thermostat can be inspected on the car like below:
Inspection when the engine is running, open the cover of coolant storage checking the surface of the coolant, if it remains calm, then the thermostat works well, otherwise it means the malfunctions. The theory go like this: when the water temperature below 70℃, The expansion cylinder of the thermostat is in contraction state, the main valve closes; when it's above 80℃, The expansion cylinder will expand and the valve opens, the internal circulation in radiator begins. If there is circulation in radiator when the temperature below 70℃, that means the valve doesn't close fully allowing the big circulation under wrong condition of low water temperature.
Inspection when the water temperature is high, when the engine just starts, the water temperature climbs up fast, when it surpasses 80℃, the temperature climbs up much slower, which means the thermostat works well, otherwise, it means it not works well.
When the temperature is between 70℃-80℃, open the radiator and radiator drainage and feel the coolant temperature difference between these two locations, if it's same hot in both location, then it means the thermostat works well. If the temperature of the radiator filler is lower and there is no coolant circulation in the inlet of the radiator or the circulation is weak, then it means the main valve can not open, there is blocking or not-full closeness existing in thermostat, it needs repairing and cleanness right away.

Hint:
The longevity of Wax thermostat is 50000km mileage in general, regular replacement required based on this time. The inspection can be done in the adjustable permanent heating equipment on testing temperature from initial temperature, full-open temperature and temperature climb-up process, replacement necessary if any above parameter not up to the standard.