Use RivaTuner. You can go into what I gather is the "card" options (the first clicky below where it tells you what card you have) and then to the system information screen. Find the option to show the video BIOS details (it will be in the list where you can select options, not the part where the information is showed). Then, find the button that is for capturing the current report, click it, and find the information about the GPU's BIOS. It will show 3 different voltages for the 6x00 cards, and it will also show the one that is currently "active" for your GPU. Thats it, you now have your cards voltage. Ask if you have any more questions.