First you need a cylinder leakdown tool. These are available from most automotive tool supply stores or shops or you can build you own. I believe it was posted on here about how to build one so just do a search.
To test it is simple. You need the tool and a constant supply of a minimum of 100 psi. Pull a spark plug and bring that cylinder to tdc. Install the gauge and hold the primary clutch. Hook up the air and hold on tight. The gauge will tell you what % of air is leaking past. You supply 100 psi and if you got 30% leakage that tells you 30 psi is leaking past the rings. Bill Cudney said, I believe that anything over 10% is an indication you need rings.
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