No expert here, but won't a $30 smart charger with capacity testing give some insight here?
Wondering this myself now.
Some advanced chargers can give you a DC IR number, but even my expensive charger is not very accurate in regards to consistent IR results, mostly because of the battery slots connections, sliding rails etc.
I’ve played with this far too many times, reseating the batteries in the slots, etc.
You then must mark the batteries and keep notes on dates, DC IR readings, so you can compare.
I have a decent AC IR Meter, the benefits of that, are most battery companies use AC IR numbers in their battery specification data sheets, and your DC IR readings are NOT convertible to AC readings.
I had fun learning and exploring, but in the end, it can become a time wasting hobby.
As even the best Molicel batteries are very affordable, I just FULLY charge them, and replace when they don’t heat up as many loads or get really old, some end up in flashlights etc.