Easiest way if the piston and sleeve are out, is to gently push the piston into the sleeve, and if it goes past the top of the sleeve it is way worn out. If you are able to push it about level it should be replaced as well. Check the forums here for a post about re-pinching, its much cheaper than a rebuild kit, and from what I have read the way the guy re-pinches the sleeve is better for the engine life as well. If the piston and sleeve are still in the engine, put it all back together, re-seal it, and use your finger to turn the flywheel, if it turns over with the plug in with little resistance its time for a rebuild/re-pinch.
-AxialLST