Check to make sure the frame is straight, and not ****. Check the swing arm for cracks or repairs. Make sure the rear shock isn't blown. Listen for any rattle coming from the clutch side cover, or a slipping clutch. Listen for a lean running bike, if possible do a leak down test on it or take some starter fluid and with it running spray the starter fluid around the case, head, cylinder, carb, case halves if the idle increases then it has an air leak.

Like all used bikes check for **** or repaired cases, especially by the front sprocket area. I always look over the wear on the sprockets and chain. Lift the back of the bike and feel for bad shock bearings, do the same with the front and steering.