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

not so easy to do.

years ago.. did this type of test on Ni-Cad packs. 20 amp load

3 grand in equipment. carbon loads are more accurate then Lights.
computer, software, analog/digital (multi channel) converter. headlight relays, carbon load.
auto off at set point. pre/during/post readings. 10 samples per second.
US department of Standards calibrated.
everything else is plus or minus 20 percent ..