First, verify that the frame is free of cracks and fractures. Inquire about repair jobs, especially ones that appear poorly done. The wheels should have no cracks or bends. Broken plastic, bent metal, worn tires, broken brake levers, and other obvious signs of wear are unacceptable.
Check that the disc brakes are not too worn. If you can see grooves where the pads touch, then the brakes have too many miles on them. The radiator should be full of coolant and not leaking. In addition, the fork seals should not leak. Oil on the front forks could mean that the seals need to be replaced.
If possible, place the bike on a stand with the rear wheel off the ground to test for give in the swingarm. More than 4 or 5 millimeters of movement means the swingarm needs new bearings. You should also check the wheel bearings and the steering head bearings for excessive movement.
Finally, ask the seller about the service history, including oil changes, and ask for receipts for any major rebuilds or repairs.