How to calculate mAh capacity of a single module

There are modules for sale on (ex: ebay) that say "4K mAh capacity" or what have you. I tried to ask people how they calculated that and they seem to say "we put it on the charger, cycle it, and the charger reports this capacity number" LOL. So...how is this number calculated by chargers, anybody's guess? Do they max out voltage then run it down to a certain voltage, and use that? Is there a common way to calculate it? Reason being is I want to buy one that kinda matches my current modules as a replacement (for a bad module) but not sure how to measure capacity of my modules to \see how they compare...

Thank you