Well, some good things to check are:
- Forks. Check for leaks and compress them to see if they are smooth.
- Head Bearings. Check to see if the handlebars are smooth and there is no play when the wheel is on the ground.
- Rear Shock. Bounce the rear shock to see if it comes back smooth and controlled.
- Rear Swingarm. Make sure there is no movement in the back wheel from side to side.
Also, just think of all the basic stuff. Make sure you start the bike yourself and ask to test ride it through all the gears to see that the clutch does not slip. Check that the brakes work smoothly. Ask about the oil that the bike has been run on. A good way to see if the bike has been wrecked hard is to look at the fins in front of the radiator and see if they are bent or warped. Oh yeah, and check to see if the light works.