Overheating

[Licensed to kill]

I have a 31 ford, all stock that tend to overheat (steaming out the overflow and cap). Im pretty sure there is no thermostat on it so, to my way of thinking, as long as the water pump is circulating the fluid it SHOULD cool (providing the rad isn't restricted). Question is, how do I determine the actual cause?. If the rad is bad, how do I know for sure. they are pretty pricey so I need to know for certain before I go the replacement route. thoughts??.