First, let's say the obvious - be careful and wear safety glasses - don't want antifreeze in your eyes. Open only a cool radiator (yep, you know all this).
Open the radiator cap when the tractor is cold, start the tractor, and then see if there are any bubbles in the radiator coolant - if so you've got a head gasket leak.
As the engine warms up coolant should start moving - if you don't see movement then the thermostat is frozen shut. But, if the thermostat was shut the tractor would overheat in a few minutes.
Make sure your fan is turning (and the water pump is pumping) - I have had one overheat when a too-long bolt was used in the cowl to hold it on and stopped the fan from turning.
And by all means make sure your radiator fins are clean - you should be able to see through the radiator.
Also make sure your timing is right - incorrect timing can cause overheating too.