1) Check to be sure you can see thru the fins, many radiator shops now don't use "real" radiator paint, and they almost fill the fin spaces
2)After running a while, shut down the engine and feel the top vs the bottom of the radiator, you should be able to tell the difference in heat
3)Undo your bottom hose and pour a gallon of water thru the fill neck and see if it rushes right out
4) pull the thermostat and do a quick check in a pan of hot water to see if it's working
5) Also with the radiator low on water, and with the cap off, do you see water movement, showing the waterpump is working?
6) with the cap off and engine running, do you see a lot of bubbles comming up? One of the signs of a leaking head gasket.
Just a few things you can try that don't cost money.....