Add here different ways to answer the above question.
With lspci
One way is to check the output of lspci:
# lspci -nnk | grep -iA2 vga
01:00.0 VGA compatible controller [0300]: NVIDIA Corporation GM107 [GeForce GTX 750 Ti] [10de:1380] (rev a2)
Subsystem: Gigabyte Technology Co., Ltd GM107 [GeForce GTX 750 Ti] [1458:3681]
Kernel driver in use: nouveau
See the 3rd line:
"Kernel driver in use: nouveau".
