You can't tell if one of these is OK just by looking and shunting it around a dealer's yard. You have to get it onto a decent piece of ground and if there's something to dig into, all the better.
I won't go into the engine, that's just the usual basic diesel engine stuff. This is only to check out the tranny.
Get it running and warm and do the following;
Engine on high idle and move the FNR very slowly to both forward and reverse and see if one track starts moving before the other. If so, that's a problem. Moving at medium slow speed, push one of the steering pedals slightly. Machine should start to turn with both tracks still driving. Push the pedal further and the inner track should stop. Push it to the floor and it should start to counterrotate. Try this in F&R, L&R. If it doesn't do any of the above, that's a problem. Move to about half throttle and repeat the tests. If it behaves differently, that's a problem.
Do the following all with the governor control at high idle.
Move the FNR to full speed. How fast does it go? If you've never driven one before, it's only going to be a subjective test, but as a guideline if you think "ho hum, here we go" then there's a problem. If you think "Jaysus, that's fast for a tracked machine!" it's probably OK. Repeat the above steering tests.
Does the engine seem to bog down excessively at full speed? Problem.
Lift the bucket a little and sight the crowd cylinder in line with an object in the distance, 50 yards or more away. The further the better. Start traveling to the object. It should travel in a straight line. Go into reverse. It should follow the track marks you've just made fairly well. Repeat the test at full speed. Does the machine veer off to one side more than a couple of feet.If so, that's a problem.
Travelling at full speed, slam the FNR into neutral. Does the machine pull up straight or lurch to one side? If it does, problem. If you haven't already, put the seat belt on and repeat using the centre brake pedal.
Travelling at full speed, drive into a (soft) pile of dirt. As the tracks begin to come under load, the engine should lug a little then stabilize at a fairly high RPM. Once the machine can't move forward any more, the tracks should continue to spin slowly with the engine revving happily away. If the engine bogs down or you have to move the FNR away from full to prevent the engine from stalling, That's a problem. Work the bucket to load up the front of the tractor. The tracks will slow down some as the engine starts to supply power to the hydraulics, but you should have to dig in pretty hard to make the tracks stall, especially if you're on worn-out three bar pads.
Try and find a hill. Park on it then set off. The machine shouldn't roll back any.
Good luck.
