Xubuntu 14.04: How do I know if I'm using my NVidia 740M when playing games?

I currently installed Wine and I'm successfully playing Diablo III. However, when I check my FPS, it's around 40 FPS. My computer is an ASUS X550LB with 8GB of RAM and an NVidia GeForce 740M. I believe the specs are alright and should run a game like Diablo III at around 50 - 60 FPS. How do I check if my NVIDIA video card is activated?

EDIT: Whenever I run

lspci | egrep '3D|VGA'

this shows up:

00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 09)

04:00.0 3D controller: NVIDIA Corporation GK208M [GeForce GT 740M] (rev ff)

Is there anything wrong there?