A stethoscope? That doesn't give you any of the information that a leakdown tester does. You generally bring the cylinder in question to the top of the compression stroke and turn on the regulator on the tester. Zero out the gage as well. The gage tells you what percentage of the air pressure is leaking out and is graduated into low, medium, and high leakage. To some extent you can learn that from a compression tester, but its not as cut and dry. The real trick is learning where you are losing your compression. While the tester is connected and the cylinder is full of air pressure listen at the exhaust. If you hear air howling there then its leaking past the exhaust valve. If you hear it at the dipstick tube its rings, carb. then intake valve, if there are bubbles in the radiator then its probably a head gasket, cracked head or block etc. leaking into a water jacket. If you already know a cylinder is bad from a compression test then you might get away with just adapting an air hose or using the one from a compression tester to fill the cylinder with air and do the same listening and looking routine. I bought the cheap HF one and it didn't steer me wrong. I was skeptical about the results but it turned out to be right in my case (I had a cracked exhaust valve). With just a wet/dry compression test I was leaning toward rings, but that was incorrect.
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