Unfortunately there is no way to do that. Take a look to this thread: Re: Battery level
That thread is about this same question. According to it, you will need an ADC to monitoring the voltage status.
I'm not sure if this question is related to your other thread: Battery and ADC Operation. But if you are using the Arduino Expansion Board you might use one analog input of the ADC to monitor the voltage level.
That would be my suggestion for you. I hope you find it helpful.