Well the only way i know if to check it is to take it out, it will be closed then.
You boil some water on the stove and drop the thermostat in and it should open, not too sure on what temperature it opens up on, but i bet if the water is like 200 degrees then it will open up.

Anyways a thermostat is probably 10 bucks so you can check it or just buy one. Wouldnt be such a bad idea if you just replaced your radiator.