As Redback pointed out the mah is the capacity of the battery and the only way to measure it is to put a known load on the battery and measure how long it takes to drain. The easiest way is to get a computer charger/discharger that will keep track of this for you, but another way is to get a resistor and a meter and do the math yourself.

If you have a 4.8 volt pack and put a meter on it and see that it has a 5v charge then you can calculate the size of the resistor you need to draw any specific current (amps). The formula known as ohms law is current = volts / resistance. So if you take a 10 ohm resistor and put it across the battery you would draw 500ma per hour (.5 amps). The math is 5V / 10Ohms = .5 amps. Make sure your resistor is up to the task of taking the heat (resistors convert energy to heat) so you need to calculate how many watts you are going to produce. That formula is volts * amps. So if you take .5 and multiply it by 5 you get 2.5 watts. A 10 watt resistor will do the job nicely.

Once you have your resistor attached to your battery pack you can make some measurements with your meter. You can put the meter inline with the resistor and set it to measure amps (as long as it is able to measure a current draw of .5) or you can measure the volts on the battery. The volts will start going down as the battery drains so you will need to know what the cut-off voltage is. I typically use 1.1v on nimh and .9V on nicad batteries. Take the number of cells and multiply it by the cut off voltage, 3.6 volts in the case of my 4.8V nicad pack.

Once your pack is at the cut off voltage disconnect the battery. If you drain your cells all of the way it may damage them, so you need to be watching this on your meter. If your pack only lasts 1 hour with a 500mah draw on it then it's only able to store 500mah of energy. If it lasts 197 minutes then you can take 197/60 to get 3.28 hours or 3.28 * 500ma = 1641.6mah.

Keep in mind that the volts will change as the pack goes dead so it's not going to be exactly 500mah draw all of the time so your numbers won't be perfectly accurate, but it will give you a good idea how this all works and is a good method to observe how the batteries hold up to any given load.

Hope that helps,
schu