For most graphics cards, GPU-Z will tell you how much a graphics card is being used, along with a variety of other useful stats.
You can download GPU-Z here.
Load up two copies of GPU-Z, and set one to monitor the second GPU. Switch both to the sensors tab, and tick the box "Continue refreshing this screen while GPU-Z is in the background".
Then load up Battlefield 3 - Set it to be windowed so you can see the GPU-Z sensors, or have the GPU-Z windows on your second monitor.
If the GPU load on both GPUs is a reasonable number, then Battlefield is using both GPUs.
Keep in mind that Windows and other background programs can load your GPUs as well. If the usage is above 10% though, its pretty much guaranteed that Battlefield 3 is using the card.
Since your cards are so powerful, also keep in mind that they might not be used fully all the time, especially if you have VSync turned on.