Terminal command to show OpenGL version?

To Check OpenGL Version,

glxinfo | grep "OpenGL version"

You will get the output as follows,

glxinfo | grep "OpenGL version"
OpenGL version string: 1.4 (2.1 Mesa 7.7.1)

Edit:

You may have better luck with modern OpenGL just grepping for "version" instead of "OpenGL version" given the differences between the core and compat profiles, as well as the various GLSL and GLES versions:

glxinfo | grep 'version'
server glx version string: 1.4
client glx version string: 1.4
GLX version: 1.4
    Max core profile version: 4.1
    Max compat profile version: 3.0
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.0
OpenGL core profile version string: 4.1 (Core Profile) Mesa 11.1.2
OpenGL core profile shading language version string: 4.10
OpenGL version string: 3.0 Mesa 11.1.2
OpenGL shading language version string: 1.30
OpenGL ES profile version string: OpenGL ES 3.0 Mesa 11.1.2
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.00

Notice that the actual version is presented by the "core profile version" (4.1), whereas the "OpenGL version" is presented as 3.0.

depending on what you're looking for:

Open GL Implementation

You can use glxinfo, from the mesa-utils package:

sudo apt-get install mesa-utils

glxinfo | grep "OpenGL version"

Development Libraries

this depends a little,

dpkg -s [package name]

will tell you version information, etc. of any package.

but you'll need to know what specific part/implementation, etc. of opengl you're interested in. i suspect, for you, it'll be:

dpkg -s libglu1-mesa