Battery capacity can't really be represented as a single number, like 42Ah, unless all the other details are specified as well. For instance, a car could be said to get 25MPG fuel economy. But if you are driving at 150MPH, that number is incorrect.
That said, if you want to measure battery capacity, you need to decide a few things:
-What represents "battery full","battery empty", and what load current or load resistance you want to discharge the battery with. This is probably something that is representative of how you will use the battery in your design. For instance, if you were designing a flashlight, you would discharge the battery using the bulb in your design (or current level that would be similar). Maybe your "battery empty" state would be a certain brightness level of the bulb, or maybe it would need to be a certain battery voltage. It depends on your design.
Once you have decided these things, you can make the "battery full" by charging it, then attach the load and make periodic measurements of the current and voltage until the "battery empty" state is reached. Do a little arithmetic and you can come up with your capacity number. You may need two AVO meters, one in series to measure current constantly, and another to measure voltage.
Depending on how sensitive your design is, you may also need to specify other things, like temperature during charge and/or discharge.