measure capacity of a battery after charged
Most battery capacity ratings are based on a 20-hour discharge time.

So: divide your rated battery capacity by 20 and calculate the resistor value that is required to achieve that current.

Note that the battery discharge voltage curve is NOT flat but the total error will be fairly small.  In other words, if you choose the resistor value to that you achieve the desired current when the battery is fully charged, that current will decay as the battery terminal voltage decays.

Now charge your cells completely and connect your chosen discharge resistor.  Then take a voltage reading on each cell every half hour or so until the cell terminal voltage reaches the end-of-life voltage.

To make this easy, you can build a simple data-logger that will automatically take those voltage readings for you and save them to a file.  You could also build the data logger such that it disconnects the load resistor when end-of-life voltage is reached so as to not damage the cell.