Usually the best way is to run the VIN but it sounds like from your description the VIN is no longer on the frame. If you know the VIN from the ownership papers or title, or if you can locate it on the steering stem portion of the frame, run it at one of the VIN identification websites. I would provide the link but I cant remember the site I normally use, I have it on my computer at home. If you need it I can provide it when I get home, but if you dont have the VIN it wont do you any good anyway. Sorry Im not much help. You could also contact Yamaha and have them run the motor number if you cant locate the VIN. Is it possible that the frame was replaced with a differenet model year?