How to accurately determine the capacity of an external battery?

Question

Batteries

How to find out the capacity of an external battery with high accuracy?
The manufacturer claims 10,000 mAh, but I would like to know more precisely, for sure there is a little less (or a little more).
Help me to understand. Thank you.

Answer 1

10,000 mA per hour, i.e. the battery at rated voltage must deliver a current of 1,000 mA for 10 hours.
if the voltage drops below the nominal after 9 hours, then the capacity will be 9,000 mAh,
11 hours - 11,000 mAh
UPD: The open circuit voltage is less than the nominal.
UPD2:
source

Answer 2

Charge and discharge, and when discharging, look at the parameters with some kind of multimeter, at least well, and calculate the capacity