yes, pull the cap off and start it up, keep an eye on the coolant as the engine warms it should start visibly start moving (my coolant runs like a raging river when the thermostat finally opens) i would also drill a 1/8 inch hole in the thermostat so air doesnt get trapped under it and cause it to overshoot before it opens.
Is the radiator new?? Have you ever put any kind of stop leak in the radiator??
