I'm having trouble with this. Perhaps you could try checking it out in the BIOS. Either that, or check out the manual or documentation that came along with it.
I have used CPU-Z, but under the section "mainboard," and on the model - it just says G10AC.
Yeah, I read that prebuilts tend to refer to the model rather than the actual motherboard. Perhaps you could check the product's website and put in a serial number. I figure there should be a serial number somewhere on the back of your desktop.
There may be a model number printed on the motherboard between PCIe slots. It is not a retail boxed motherboard, the previous link contains the BIOS updates, drivers and utilities.
http://www.asus.com/Desktops/G10AC/