[AndrewJ]

I tested my 12v batteries by hooking them up to a load (in my case my car, with headlights on) and started my stopwatch.
Once the clock was running, I recorded the voltage (with my DMM) every 30 seconds.
When comparing the voltages you could clearly see which batteries where good as they took longer to drop voltage, and they stabilized at a higher voltage.

I think Darin mentioned that one should use a test load that somewhat closely matches the intended load of your batteries.
Since I'm using mine to run my car's electrical system, I figured the (relatively) high draw of the high-beams would best mimic the overall draw of the cars electrical system.