Rating your battery packs will show you how strong they are, the higher the "working MHa" rating the long the pack will run. You can also go by the "dis-charge time", again the higher the number the better.
I use old car headlights and tail-lights myself. I use three 1157 tail-lights (3 dim elements, and one bright element), one round headlight (highbeam [from 4 lamp system, my old Honda]) and on rectangular (highbeam [from 2 lamp system, my old Pontiac] So in total I have to highbeam lamps, 3 dim & 1 bright elements for 1157 tail-light bulbs. That give me a 9.6amp load. Old car lights are great for discharge loads, they're cheap and in my case, taken out of the car when one element of the bulb has failed and the other remains working. You can also buy resistor loads for hobby shops. (Team Timm plans on offering a pre-made load later this year)
You need a volt-meter that can read the amps load in a circuit. I use an old Radio Shack on the can handle up to 10 amps. It goes between the battery and the load and tell you the amps being pulled. Make sure it one that can handle at least 10 amps, many don't go that high.
Cutoff voltage is an industry standard for rating battery packs, .9 volts per cell. This allows a standard level for rating powers. When a cell drops below .9 volts that's the end of the rating cycle. The the pack rating is figured from the discharge time and load.
The outputs the really count are the "working" MHa. This is how much power the pack holds. The average voltage: the higher the better. Higher voltage means a faster car. The discharge time in seconds: This is hold long the pack puts out power before dropping below .9 volts per cell. The industry standard discharge load is 20 amps, but it hard to come up with a 20 amp load, (like my odd ball 9.6 amps), so the R/C Helper program figures out what the discharge time would have been if the load had been 20 amps instead of what ever it was. This lets you compare batteries fairly against other rating under different loads.
I suggest any of the R/C car magazines, there are also a few books out, you might want to check you local library for them. I'm not sure offhand what to look under, try "radio control" or "remote control" or "models". There are other magazines out there, a number of British ones.