Kubota 2502H, HSI Pinnacle grapple, LP 48" forks, LP 60" BB, KK tiller 60", KK middle buster, Pat's QH
I would start with the most obvious things such as a clogged air filter or radiator screen/fins, fluid levels as well as condition of oil/coolant, fan belt, collapsed radiator hoses. And then move on to more complicated tests.
Does it overheat all the time or just when performing certain tasks? You could be asking the tractor to do more than it was designed to do. You may not be operating it in its optimum power RPM. Perhaps you're trying to do the work in too high a gear. Many things can cause a tractor to overheat which are not cooling system related.