look at the chain, listen for knocks when you ride it, how much? make sure the clutch and brakes are good, its really listening to it, i would even take compression tester with you, lol, get rid of the oil injection if not already done.
just check all your bearings buy a compression tester its worth the 20 bucks if your looking to buy a bike.. check compression should be 120 or better.. anything lower tells you its on its way to a rebuild.. look at the plug.. you want it to be mocha color or cardboard color.. feel the exhaust by the head is it warm/hot. did they prestart it before you got there?? put the plug back in turn the switch on and key if it has one. kick it how many kicks does it take?? when it fires listen to it.. then take it for a run.. look for smoke. it will smoke im talking alot of smoke from the exhaust. if it runs good.. pull the plug again when you get back and see what it looks like after running it.. some thing to look for.. hope that helps