I had found that if you turn the heater on and the temperature drops. It was the radiator in my few cases. I just do not feel comforatable with this test as being really good at all.

May depend on if circulation through the heater core is active if there is little cooling system flow. I suppose if it did not pull the temperature down might mean something as well.

Some radiator shops may offer a flow test or the use of that hand held temperature sensor might be a good ideal as mentioned.

I would do the old thermostat test in a pan of hot water with a thermometer. If the old thermostat proves bad there is a fair chance the new one is as well.