Answer:
The most direct test is to remove the thermostat and place it into a pot of boiling water. If it opens when hot and closes after cooling, then you know the thermostat is working. If the temp gauge stays below 195 degrees, it's likely that the thermostat is stuck open and not allowing the engine to warm fully.
In addition, check your coolant level with the engine off. Ensure that the radiator is full to the top and the coolant level reaches the cold mark on the overflow tank. Too little coolant can also cause no heat because the coolant won't circulate properly. The heater core may be empty of coolant or is simply not receiving any warmed coolant due to air pockets in the system.
If the coolant is way low, remove the cap and top off the radiator. Park uphill if possible so that trapped air heads to the radiator cap. With the cap off, start the car. The coolant level will probably fall. Continue topping it off until the level is steady. Replace the radiator cap and adjust the overflow tank level. Investigate leaks if your coolant is unusually low.