I have used a HF compression gage kit on my 3 OM617.952 engines. I haven't tested the gage against another, but seems reasonable. I read ~250 psig in most cyl when my 85 engine needed a block heater to start on cold mornings and had much blow-by. The replacement engine measured >400 psig in all cyl and showed almost no blow-by. My 1984 engine has a few cyl in the 300's and noticeable puffing from the PCV port. Thus, you could pretty much judge the rings from the blow-by. I recall paying ~$35 on sale for the kit. In all cases, I used the injector holes. Note that the results depend on the adapter displacing exactly the same volume as an injector. If using the glow-plug holes, I would first remove all the injector tubes for easier access.

I don't see how an endoscope could view the valves without first removing the prechamber, which I understand is non-trivial. Perhaps you could use a compression test kit to rig a leak-down setup and apply air pressure and see where a cyl leaks to (intake, exhaust, or crankcase), as is done for gas engines.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans