If you use the 'Display' button, each press will cycle through the data.
The 'Display' button will show either 'mA' for milliamps, that is the rate of charge or discharge. Then there is 'V' for voltage that the battery is currently at. Then'mAh' for milliAmp hours, this is what you want, it is the capacity of the battery .
You will know when it is done discharging by looking at the 'mA', it will show zero.
And when charging a battery, the bottom of the display shows 'Full', also the 'mA' will show zero as it is no longer charging, but don't really care about 'mA' when charging because the screen will show 'Full' When done.
When discharging a battery, if it had some charge to it, or it was fully charged and you discharge it, then the 'mAh' will show the capacity that battery had once it finishes discharging. And if you then charge it, when it finishes the 'mAh' should be close to what the discharge 'mAh', assuming the battery was fully charged to start with.