Good responses, I'll add a few more...
If the tractor has hydraulic levers on the dash, there may be weep holes in the left side of the block- at least there is on diesels. Make sure no antifreeze is oozing from the weep holes.
Check out the power steering. Turn the wheel quickly from left to right, and then back. Does it take longer for the wheels to turn than it does to move the wheel? There should be little or no delay, especially after the first cycle.
Tx Jim mentioned wear on the pedals. that can be a good indicator of total use. Drawbar wear is not a very good indicator. Drawbars do get replaced, and one would expect that in the lifespan of a 40-50 year old horse.
Look for coolant in the oil, leaks anywhere, etc.