It's very easy to check your DX version,
In the same place, ie, Start> DxDiag, go to the second tab, called 'Display'.
On the Right hand side, under the 'Drivers' section area, the last line, would say something like Driver Model : 'WDDM 1.2'.
WDDM 1.2 => DX 11.1
WDDM 1.3 => DX 11.2
That's it; although if you're sure you're running Windows 8.1 on a GCN card, with the latest drivers, there's no doubt that you'll be running 11.2 !
Cheers!