well no, the card itself doesn't read voltages. if it's a GT, it will always report 1.2V if anything. the PCB is the same as the GTX's PCB, except with a few things taken off to remove the option to change voltage in software (usually a BIOS modification)
by performing a volt mod, the card itself doesn't "know" that it's running at a higher voltage. as far as it knows, it's still running at 1.2V. but the mod just lets in more voltage
anyway, what I did with my card is, instead of doing a pencil mod, I actually replaced the resistors with ones of the desired resistance.
doing that requires a bit of skill in soldering.