Start with the small, easy stuff and work your way "upwards"...
http://fc3spro.com/TECH/PROBLEMS/P1/...OT/coolant.htm
If the thermostat is stuck, check the core of the radiator with your hand CAREFULLY.
The core of the radiator should be evenly hot when the thermostat opens.
If the bottom of the radiator (by the lower radiator hose) isn't as hot at the top (by the upper radiator hose) this is a possible sign the thermostat is bad.
The above also would indicate a blockage in the radiator core and / or something wrong with the water pump.
Easy test to see if the thermostat is working or not - throw it in a pot of boiling water.
-Ted