Does it sound normal when initially starting up? ie; if it has a torched piston it will noticably miss a beat when cranking over prior to startup.
Secondly, when idling remove the oil cap and observe the amount of gas escaping. Not necessarily the pressure (as it'll go elsewhere when you block it off) but the volume of gas. If it's like an exhaust pipe you have a blown out piston (which is quite common)