Is there a real-world test of a battery's true capacity?

do your research right.

Used tested and calibrated equipment... some are able to be Rented...

do a search.. use Real accurate equipment..

not low cost ..not calibrated... almost worthless stuff.

accurate cost money for a reason.

consider contacting the engineering department of your choice battery manufacture.
Yuasa, Johnson Controls, Exide. or ??? many documents are available.