If the pot of water is simmering, it is close to 180 degrees,, a rolling boil is close to 210 degrees. Most automotive thermostats open at 180-210 degrees. You can buy a thermometer at the grocery store for a fraction of the price of a thermostat, one for cooking meat should indicate the proper range of temperatures for nearly all thermostats. A thermostat that fully opens at 140-160 and closes below that temperature wont cause much trouble for the engine, the only side effect would be an insignificant decrease in fuel economy and heater that doesn't blow as hot. Nearly all malfunctioning thermostats will get stuck open or closed.
The proper way to test the thermostat is to sit it in cold water and raise the water temperature and wait for it to open, just to immerse in in hot water may or may not open correctly giving you an incorrect result, it should be fully immersed as well
