With the radiator cap off, warm the engine and verify that the coolant is circulating inside the radiator. I keep a bi-mettalic thermometer with a 12" stem for verifying such things. You can probably find one at your local grocery store or cooking store. Just make sure the range is well over what you want to measure because the dial types are typically not accurate below 20% or above 80% of the scale.

http://www.ertco.com/bimetallic.html