Measure different Vcc using 1.1V bandgap
The internal reference won't change much. That's why it's a reference.

Running this code on my Atmega328P:

void setup ()
{
  ADMUX = bit (REFS0) | bit (REFS1);  // Internal 1.1V reference
}
void loop () { }


That outputs the reference voltage on Aref. I measured it as follows with different values for Vcc:

Vcc     Aref
------------
5.00   1.085
4.00   1.081
3.30   1.080
3.00   1.079


So you can see that it retains, more or less, the same voltage regardless of Vcc. In any case, there is no way of measuring Aref, unless you have another reference voltage around to compare it to.