I've come up with a bulletproof way to test battery capacity. It does only work for 12 volts, but since I use a serial / parallel jumper plug to my 48V pack for charging, it is easy to tap the same pack for 12 volts as well.
I take a conventional 12V inverter. The one I have is 400Watts capacity. I hook that to my test battery with heavy capacity cabling. I then plug a "Kill-a-Watt" power meter into the 120V output of the inverter. I then plug a 275Watt resistive heater into the Kill-a-Watt.
The Kill-a-Watt will then monitor and total the electrical power used by the heater. A 275 Watt load means 2.3 Amps @ 120 Volts, which requires that the inverter draw 27.5 amps from the 12V pack. (Of course, the inefficiency in the inverter means that the actual draw on the battery is a little greater (by this source, that would amount to 6% more.