If you use a series resistor voltage divider circuit, could you bring the battery voltage within the range of the analog inputs on the Arduino, and then just use the analog read function to check the battery level?
This is the easiest way. Use a 2:1 divider for a 9v battery.
Use large values of resistor, so they don't drain the battery.
Something like 100KΩ should be good. Use a 100KΩ between ADC input and ground. Use a 100KΩ between ADC input and battery +.
(make sure the ADC->Ground resistor is in place before you do anything else or you may nuke your chip).
The ADC will read between 0 and 4.5V (ish) from the divider.