Look for the 'usual' stuff.
Oil in water, water in oil, leaks under tractor, or evidence ov leaks in the paint. Look for evidence of repair.. Good repairs are ok. Bad or questionable repairs are a warning sign... for instance.. RTV extruded out around a casting mating surface may mean a proper gasket wasn't used.. etc.
Check whgeel hubs for play by rocking the tractor side to side. Look for play inthe steering members and linkages/steering box. Look at the wear or scuff patterns on the tires.
Drive tractor in all ranges and all speeds. Run it long enough to see it's hot idle oil pressure. Start it when it is could to see how easy it is to start. Verify that it charges, and that instruments work. Check clutch pedal travle, and even the wear on the clutch pedal itself. Check the brakes. Check the lift.. make sure it is strong. Try to test it with an implement on it, lift it, and then turn the tractor off and see how long the lift stays up. If possible also try out a PTO implement with it.
If a private seller.. look at other things he owns as a 'guide' to see how well he maintains things.
Good luck.
Soundguy