To verify capacity yourself
ideally per cell but that may not be practical
next best, smallest sub-pack voltage that is practical
Worst case each pack as a whole at the full-pck level voltage.
Buy a suitable dummy load tester, ideally one that outputs data (Voltage, Amps, Watts derived, over time) that a PC program can analyze and render graphs.
Or if poor, gang DC headlights, LEDs or inverters+ bar heaters, hair dryers, or just "brake load" resistors
you are looking for handling the ballpark right voltage, drawing between .05C (twenty hour rate) up to say 0.3-4C.
The key is keeping the current drawn as precisely constant as possible
from the 100% SoC point, down to say 3.0Vpc. Then all that's needed is a precise timer, and an app that takes say 200 photos automatically over the time period, if you want those graphs without a datalogger.
Or if your CC is only rough, get a wattmeter that counts coulombs to give you cumulative Ah.
If the cells turn out to have lost over 20-25% of rated capacity, most companies would call them EoL scrap
and the potential risk of "thermal runaway" (boom bad!) is exponentially higher than when "as new"