Before you open up the system to install the thermostat, cover about 2/3 of the surface area of the radiator with a piece of cardboard. When you run the engine, the temperature should rise because of the restricted airflow. Don't run it for long that way, but you should see a pretty definite temperature rise. Another trick we used to use to prevent an air lock was to leave the upper radiator hose disconnected during the radiator fill-up to be able to see when the block, heads, intake manifold, etc were full.
Jerry
