Usually the way to tell if it is the water pump or a stuck thermostat is if your heater will blow warm or cold. If it is a water pump, the heater will be cold since the water pump will not circulate coolant anywhere. If it is the thermostat, the heater will be hot because the thermostat only restricts circulation to the radiator but still permits circulation thru the rest of the system.
In short, if the heater will not get hot, it is time to check the water pump.
