How can I see which video card on my Optimus-enabled machine is being used?

The command glxinfo displays which video adapter is used for OpenGL graphics. Look for a line similar to the following:

OpenGL renderer string: Mesa DRI Intel(R) Ivybridge Mobile

The above is displayed on my laptop even though it has Nvidia Optimus discrete GPU. This means that it is still using the integrated Intel GPU for 3D graphics.

After setting up bumblebee I run optirun glxinfo and can see that the system now uses the Nvidia GPU, but only when running commands with optirun:

OpenGL renderer string: GeForce GT 540M/PCIe/SSE2

In the end I found out that the Nvidia GPU performs actually worse through bumblebee than with the integraded Intel GPU enabled with DRI_PRIME=1 setting. I will try again when/if there is a better solution in future.

You should use your package manager to install the Nvidia drivers because when an update comes down the pipe it makes it easier to upgrade the kernel and still keep your nvidia driver, or even get nvidia driver updates automatically.

If you have the other nvidia driver installed you will need to manually uninstall it. At least in my experience that is the most successful method. Those details are outlined in the read me file that came with the driver.

If you want to use the tested drivers that work well under Ubuntu then run this command

sudo apt-get install nvidia-current-updates

That should do it. If it is working successfully you should have an Nvidia control panel you can locate that in the dash.

It should give you your nvidia system temperature and other facts about your card.

hope this helps.

One command is

lspci 

which produces, beneath other things, for example on my system:

00:00.0 Host bridge: Intel Corporation Mobile 945GM/PM/GMS, 943/940GML and 945GT Express Memory Controller Hub (rev 03)
00:02.0 VGA compatible controller: Intel Corporation Mobile 945GM/GMS, 943/940GML Express Integrated Graphics Controller (rev 03)
00:02.1 Display controller: Intel Corporation Mobile 945GM/GMS/GME, 943/940GML Express Integrated Graphics Controller (rev 03)

a similar tool is lshw (list hardware):

sudo lshw -class display

In Xfce, there is an entry in the settings menu, 'additional driver', where you can look, if a driver like the Nvidia driver is installed and active, and where you can search for such a driver. The name of the menu might vary on your system.