E6420 Lists Both Intel GPU and nVidia GPU in the Device Manager. Which is being used?

Google "NVIDIA Optimus".  Basically, the system defaults to the Intel GPU in order to save battery life when there is no graphic-intensive work going on that would be enhanced by having the NVIDIA GPU active.  In many systems it's also the only GPU physically wired to the display outputs.  When the performance demand increases, e.g .when playing a game, then the NVIDIA GPU is activated and acts as a render-only device, doing the heavy lifting and then passing completed frames to the Intel GPU for output to the displays themselves.  In general, this system works well (and it continues to get better with newer drivers), but there are times when the automatic mechanism does not select the desired GPU.  In those cases, as ejn63 said, you can use the NVIDIA Control Panel to force a particular GPU to be used when a certain application is executed.  You may also notice that if you right-click an application shortcut (at least on the desktop, not sure about the Start menu), you'll see an option that says "Run with graphics processor" and then allows you to select the Intel or NVIDIA GPU.  That is a handy way to choose a specific GPU on a faster, per-execution basis as compared to storing a persistent profile in NVIDIA Control Panel.

However, I would recommend that you just use your system and not worry too much about that unless you see poor performance.  In that case, try manually forcing the application to run on the NVIDIA GPU, and if that improves things, then set a profile if needed. Otherwise, let the system do what it's designed to do, because in most cases it's likely to will do a better job than you. :)