How can I tell that I'm gaming with both my GPUs?

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.

In the Nvidia Control Panel, there is an option to "Show SLI visual indicators".

It will show a ugly green bar all over the left side of your screen, but you will definitely know if its working or not.

This is especially useful if you only have one monitor.