You should see lots of movement inside the radiator once the t-stat opens. If that's not happening the these things:

1) Remove the hose from the thermostat housing and look inside. Is the thermostat pointy side up? If it's upside down (spring end up) it could restrict the water flow.

2) Pop off the upper hose and start the engine. Make sure the radiator has water in it higher than the water pump. Does water come out once the engine reaches temperature? If not, there's some sort of restriction in the engine's cooling passages such as the thermostat not opening.

3) Pull the spark plugs. Are any of them cleaner than the others? If so, you probably have a head gasket that is allowing exhaust to pass into the cooling system. You usually see bubbles in the coolant when that happens.

Let us know what you find with these things and we can take it from there.