Easy to tell. Both hoses to the heater core should be hot. If one them is not very hot, that means there is little to no flow through the heater core.

How do you tell if your thermostat is working? Start the engine cold in the morning with the radiator cap off. If you see flow through the radiator by looking down where the cap was, the thermostat is stuck open. You will also feel the upper radiator hose starting to get warm after just a few minutes of running the engine.

If the thermostat is working, you will see no water flow in the radiator with the cap off, and the upper radiator hose will remain cold till the thermostat suddenly opens.