How to get the version of the nvidia driver

Seems to work! Thanks @cas (see comments on question). I installed bumblebee (from backports, since that's where I installed nvidia-driver and dependents from, as per the wiki):

$ sudo aptitude install -t jessie-backports bumblebee bumblebee-nvidia

and rebooted to make sure everything was working (you probably don't even need to restart the X server, but I was feeling charitable). NB if you haven't yet installed the nvidia driver, I believe this would also pull that in? I haven't tested that though. It may instead pull in nouveau. If when you run primusrun glxinfo you don't see "nvidia" anywhere, this may be what's happened; in that case presumably you can just install debian as per the wiki and it should work (you'd need to reboot then for definite, since it installs a new kernel module).

Then used glxinfo and glxgears to test.

$ glxinfo | grep OpenGL
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) Haswell Mobile 
OpenGL core profile version string: 3.3 (Core Profile) Mesa 10.3.2
OpenGL core profile shading language version string: 3.30
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile
OpenGL core profile extensions:
OpenGL version string: 3.0 Mesa 10.3.2
OpenGL shading language version string: 1.30
OpenGL context flags: (none)
OpenGL extensions:
OpenGL ES profile version string: OpenGL ES 3.0 Mesa 10.3.2
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.0
OpenGL ES profile extensions:

Note that running glxinfo bare uses the integrated intel graphics (I think mesa used to mean software opengl? I don't know what it means now. But it's clear it's NOT using the discrete card).

$ primusrun glxinfo | grep OpenGL
OpenGL vendor string: NVIDIA Corporation
OpenGL renderer string: GeForce 940M/PCIe/SSE2
OpenGL core profile version string: 4.4.0 NVIDIA 352.79
OpenGL core profile shading language version string: 4.40 NVIDIA via Cg compiler
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile
OpenGL core profile extensions:
OpenGL version string: 4.5.0 NVIDIA 352.79
OpenGL shading language version string: 4.50 NVIDIA
OpenGL context flags: (none)
OpenGL profile mask: (none)
OpenGL extensions:

Success!