Plz check sprocket, drive chain, brakes, clutch lever, frame, brake fluid oil, suspension, bettery, rust, leaks and tires, all this things can give you clear idea of how to owner mainteined the bike and if the motorcycle has met any accident or not.
If you are not thet expert, ask your mechanic to check all this things carefully.
If you see any damage on frame just don’t buy it and always try to buy motorcycle which is stock, Avoid bikes with after market parts.
After you have done this, ask the owner or dealer for the papers and check it thoroughly.