How to measure the supply voltage of Arduino without using an analog pin
Yes, you can do it.

You can measure the voltage of the internal (approximately) 1.1V reference voltage using the ADC and use the results of that to calculate what the reference voltage you used (VCC) must have been to get the results you did.

This is a function I commonly use:

long readVcc() {
  long result;
  // Read 1.1V reference against AVcc
  ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
  delay(2); // Wait for Vref to settle
  ADCSRA |= _BV(ADSC); // Convert
  while (bit_is_set(ADCSRA,ADSC));
  result = ADCL;
  result |= ADCH&lt;&lt;8;
  result = 1125300L / result; // Back-calculate AVcc in mV
  return result;
}


Depending on how close to 1.1V your internal reference voltage is, you may need to adjust the value 1125300L in the code above.  Measure the actual VCC voltage and adjust that value until it results in the same voltage being returned (multiplied by 1000 since that function returns millivolts).

However, as Edgar Bonet mentions, you don't need to care about the supply voltage in your situation, since your thermistor readings are just a ratio that input voltage, whatever it is. Change the input voltage and you will get the same results, since the ADC returns a value between 0 and Vcc in 1024 steps. With different supply voltages you still get 1024 steps.