ATTINY85 measuring internal voltage
Setting REFS[2:0] to 0bX00 will use VCC as the voltage reference, and setting MUX[3:0] to 0b1100 will use the internal bandgap voltage as the voltage to measure (see &sect;17.13 from the datasheet). From there, the full range of 1023 will tell you what VCC is in relation to the bandgap voltage. So take (1.1*10*1023=) 11253 (or an appropriately scaled equivalent) and divide it by the measured value in order to get the approximate value of VCC in tenths of volts.