How do I know if XPS is using the correct GPU?

"Default" simply means the nVidia GPU will be used everywhere supported -- remember this is a muxless system - software controls the GPU, and ALL video data passes through the Intel GPU on its way to the display panel (the nVidia GPU has no physical connection to the LCD panel).  

That means the Intel GPU will always show active.

If your application supports software-controlled hybrid video, the nVidia GPU will be used.  There are applications that do not support this setup - they require a true, discrete GPU this system doesn't have.

These are high-end applications and this is not a high performance graphics system - check with the publishers of the software packages in question to see if they do support running on a system without a discrete GPU option.

 

Right-click the application shortcut and you should see a “Run on graphics processor” option. Compare the performance of each mode to what happens when you lunch normally; the difference should be significant. If you find that an application doesn’t switch correctly, you can use NVIDIA Control Panel to specify the desired GPU for any application.