I use something called a Sirius Super Test. It works really well with no guesswork. It works on 4, 5, and 8 cell battery packs with choice of 3 different loads and shuts itself down when the battery pack reaches 1 volt/cell. I found a link that you can follow to look at one. I have run my own tests on the unit to check its sensitivity and repatability. ie. battery pack juast coming off the charger vs. the same pack sitting for 2 days after coming off the charger. As far as I can tell the unit is "very accurate" and I have used mine for about 3 years now.

http://www.SiriusElectronics.com/supr_tst.htm