Try the freeware HWiNFO64. Download it from here:
http://www.hwinfo.com/download.php
The Portable version doesn't install anything onto your system whereas the Installer version is like installing any other application.
I can't guarantee that it will work with your specific system because I didn't see the nForce 790i Ultra SLI chipset on its Chipset support list, only the nVidia nForce 720i/730i/740i/760i SLI, GeForce 9xxx (MCP79/7A), ION, MCP89 are listed. If the sensor chip on your motherboard is supported then there's a great likelihood that the voltages will be correctly displayed.