The app has an amp hours used meter, so if you have measured the batteries capacity you can have an idea of how much energy you have left. If you want to add more to it the app is open source and any code submissions are gratefully received!
Actually measuring a batteries capacity is a more involved task. Normally done on a benchtop with a power sink that can replicate the sort of current the car uses through a race (or for a more advanced test record current through a race and re-play that on your battery test rig), then measure the voltage and current of the battery as it discharges. The eChook could be modified to run a battery test as it has Current and Voltage logging, and a PWM output if, but it would require some coding.