Aircraft mechanics use a leak down tester to determine an engine's condition at every annual inspection. It's used by bringing the cylinder up on TDC on the compression stroke, and applying 80 psi to the unit by adjusting the regulator on the left. The air goes through a metered orifice, and the second gauge shows the amount of leakage.
Naturally, a perfect cylinder will show 80/80. That almost never happens. 10% leakage would be 80/72, and is fine. Investigation begins when a cylinder is under 80/70, although that is not necessarily bad. The good thing about this is this:
If you listen to the exhaust, you can tell whether an exhaust valve is leaking, at the carb for intake valve, or at the engine breather, rings.
Simple, no? And a much better way of ascertaining engine condition than a "compression gauge."
It's $50-100 at aircraft suppliers.