How to know whather Dirt 2 is DX11 or DX9

if you have vista or W7, a DX11-GPU and a DX11-game it will play on DX11 natively
if you haven't changed it in the game-options (in the rare possibility that you can choose that) it will be DX11.

you supposed to see the difference

Make sure all options are set to their highest settings.

Basically, if your running at the 30-40 FPS range, you're running DX11. If you're above 60FPS, your running DX9.

As for visually, the differences in Dirt2 are minimal anyways, and probably not worth the performance loss.

You could also try running Rivatuners OSD framerate counter.

yeah use fraps

change the relevant in game settings to ultra. the ultra settings denote the DX11 changes (not on all options, but the DX11 capable ones). those settings wont be selectable on a non DX11 card.