Wheel motors. Unfortunately, that's not something you can really check for until one pukes. However, here's one way to do a simple check... Fire the machine up, warm it up and drive it around. If there's any type of surge to the system there's something wrong. You'll just have to feel that by the seat of your pants. Then, if that checks out OK, and this next part will sound crazy, with the machine setting still, and with a clear run in front of it, shift it up to road gear, set the parking brake, throttle up to operating RPMs, then stroke the hydrostat full forward. If everything's in working order as it should be, it will put the hydros into bypass and the engine will still run. If it lugs the engine down and kills it you've got problems. Or if the brakes don't hold and your reaction times aren't good, you might have a different set of problems

Also, they don't start worth a hoot in the cold. If it's under 50 degrees it might start, but it's going to sound terrible. Even w/ the blockheater plugged in. It's probably also got a tanke heater for the hydro tank, and that's what's important to plug in too b/c of the amount of oil/power the hydros take. Lots of lug on a cold engine w/ cold oil.

Other than that, all the normal things to just look over.