First Pull the plugs and look at them... If one is melted then you have a melted piston. If not then a Compression test in need here. If you dont have one then you can see if you have a blown head gasket. Pull the radiator cap; and with a flashlight, start the bike and look over to the return side of the radiator as much as you can and see if you can see bubbles coming out of the fluid. If so then you have a F'ed up head gasket.