Which linux kernel driver is being used: nvidia or nouveau?

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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".
In a chroot environment, the output would be that of the host linux kernel, not the one you would get if you directly booted into that environment.

Package installed

apt: dpkg

On Debian, Ubuntu and other derivatives, see what package is installed with dpkg:

# dpkg -l | grep -i nvidia
ii  libnvtt2                              2.0.8-1+dfsg-2~11.04~wfg1                    NVIDIA Texture Tools
ii  nvidia-common                                                    Find obsolete NVIDIA drivers
ii  nvidia-current                        285.05.09-natty~ppa1                         NVIDIA binary Xorg driver, kernel module and VDPAU library
ii  nvidia-settings                       285.05.09-natty~ppa1                         Tool of configuring the NVIDIA graphics driver

Kernel module installed

Kernel modules are installed in /lib/modules
See http://linux.overshoot.tv/lib/modules
nvidiafb is not the proprietary nvidia driver, but the framebuffer driver for nVidia graphics chipset.