you can do a compression test. The numbers you get really aren't as important as if they're all within 10% of each other. If you get low results (more than 10% diff. from highest psi) on a few cyclinders, you can throw some oil in there (not much, prob. a cap full?) through the spark plug hole. Re test...if compression goes up w/ oil added, then you know it's bad rings - the oil fills in the gaps left by the rings. You might search for info on a "leakdown test," which is a test where you input air into the cyclinder and see how much of it stays...I believe that's basically what you do, but it's supposed to be a better test because it has nothing to do w/ valves opening, etc.