Drive it around. Go max speed forward and max speed back. Forward should be much faster than reverse. The 325 I picked up had an issue where the forward speed was limited. I eventually found the problem, which required something like a $2 part and lots of elbow greece.
Also, check the steering for looseness or tightness. My steering was pretty loose. It's tight now though, although I may want to make it tighter yet.
Make sure the PTO clutch works by engaging the mower deck. My PTO clutch had to be replaced, but I knew that when I picked the tractor up.
My 325 also had quite a bit of cosmetic issues, such as busted hood and torn seat. But it runs well.
The 325 is a nice tractor. If you get this one, be sure to post pics.