I could think of a few different ways to see what your battery's max capacity is.
You could fully deplete the battery (almost dead) and put it on a charger to see how long it takes to charge up, at the given current rate you set. If you can't set the charge current, then you'll have to see what your charger puts out, at it's fixed rate, and do the math. For example, a charger that puts out 5 amps, for 30 mins, will have put about 2500 mili-amps, or 2.5 amp hours of charge back into the battery pack. If your pack is rated for 5000 mili-amps, or 5 amp hours of capacity, and it only wants to take half that, from an almost fully depleted state, then it's lost a lot of it's capacity.
Some fancy chargers will actually display the amount of charge a pack has taken, in mili-amps. That's the easiest way to know how healthy your pack is.
Still not sure? Take the pack, fresh off the charger, and hook it to a load that you can measure the current with. For example, hook it to a motor and measure the current draw of the motor. A motor that draws 5 amps, continuous, under load, will be your known value. If your battery pack can continuously support that 5 amp draw for 30 mins, then you know it's max capacity is only 2500 mili-amps, or 2.5 amp hours.
Another easy way, but not cheap way, to know what your battery's condition is, would be to get a current monitor gauge hooked up in series with it. Monitor the current draw, of the load that way, and see how long the pack lasts. Then it's simple math to solve for the battery capacity. Here's the Watt meter I have.
http://www3.towerhobbies.com/cgi-bin...&I=LXLMV0&P=ML You can do the same thing with a digital multi-meter, wired in series with the load, or current draw.
Or, if your battery charger has a discharge feature, you can run the battery down using the charger, and set the discharge current rate. How long it takes to discharge will tell you the capacity too.
If your batteries are old, and you haven't cycled them much, chances are they have lost a lot of capacity from just sitting around. You might not be able to bring them back to life. Even a battery that is heathy, and is cycled on a regular basis, will continue to loose capacity, the more it is used. A little at a time, but eventually it will suffer a noticable loss.
Sometimes, you might think the battery is the problem, when in fact a battery charger can be the problem instead. Cheap chargers sometimes false peak, before the pack has reached full capacity. It makes it look like the pack is bad, when it really isn't.
Hope this helps