One way of finding out is by performing a leak down test. You can do this if you have some way of putting compressed air into the alleged bad cylinder at TDC. I use a compression gauge hose that has an air line connector on it. I screw the compression guage hose into the spark plug hole of the suspected bad cylinder. With that cylinder at TDC (you may have to pull the valve cover to make sure you're at TDC for that cylinder), hook up the hose to a compressor and pressurize the cylinder taking care that the crankshaft doesn't move. You may need to use a breaker bar wedged against the frame to keep the crank still. Once the cylinder has been pressurized, you can listen to the intake, exhaust pipe, and valve cover breather for leakage. If you hear air escaping from the exhaust pipe, you have a burned/bent exhaust valve (provided the valves are closed). If you hear air escaping from the intake/throttle body/afm/air filter, you have a bent/burned intake valve. If you hear excessive air coming from the valve cover breather or crankcase breather, you have bad rings.

Ian

 

sounds like a plan, its weird some days its bad (hot days) others its hardly noticable.