I used an analog voltage divider like this one from Elec Freaks:
http://www.elecfreaks.com/store/octopus-analog-voltage-divider-brick-obvoltage-p-429.html
You can make one yourself but the brick is cheap and easy to implement. The voltage it reads from the analog pin is multiplied by a constant to get the actual voltage. I calibrated the constant by comparing the brick output to my multi-meter. I'm not sure why but I got different readings when I ran it powered by USB versus being powered by the jack. I put a small 9 volt power regulator between the batteries and the Arduino power supply so that it would be over powered.
A conceptual drawing of the circuitry is attached. Pins used by the LCD controller have a blue line across them. I attached all of my wires to a prototype board that is sandwiched between the Arduino board and the LCD controller.
