You can measure the internal 1.1V reference voltage relative the AVcc (which is normally connect to Vcc). If the supply voltage is 5V you would get a reading of about 225 ((1.1/5.0)*1024) if the supply voltage drops to 4V you will get a reading of about 281 ((1.1/4.0)*1024).
Unfortunately you can't use "analogRead()" to read the internal voltage standard. You will have to set the ADMUX register to 14 by direct port manipulation. Here is a tutorial that covers the ADC: http://fivefive.wordpress.com/2008/01/23/avr-tutorials-adc/