How do I test my thermostat?

Hello,
A thermostat usually fail in 2 different ways.
First it could be stuck closed. If it is stuck closed the engine could overheat, as it prevents coolant from flowing thru the radiator to cool down.
Second it could be stock open. If it is stuck open then the engine may not reach its operating temperature. When this happens the heater will not blow enough hot air.

Your engine operating temperature is 195 deg. F.
If you have temperature gauge you can check the top and the bottom radiator hose if one is significantly cooler then the other then you might have a stuck closed thermostat.

Here are couple of helpful articles:

https://www.2carpros.com/articles/how-an-engine-thermostat-works
https://www.2carpros.com/articles/replace-thermostat

Videos of thermostat replacement procedure:

https://youtu.be/RIC6Z4D6Q3E
https://youtu.be/EZ4luHCzBeo

Please let me know of any questions.
Thank you.