You can use python -V (et al.) to show you the version of Python that the python command resolves to. If that's all you need, you're done. But to see every version of python in your system takes a bit more.

In Ubuntu we can check the resolution with readlink -f $(which python). In default cases in 14.04 this will simply point to /usr/bin/python2.7.

We can chain this in to show the version of that version of Python:

$ readlink -f $(which python) | xargs -I % sh -c 'echo -n "%: "; % -V'

/usr/bin/python2.7: Python 2.7.6

But this is still only telling us what our current python resolution is. If we were in a Virtualenv (a common Python stack management system) python might resolve to a different version:

$ readlink -f $(which python) | xargs -I % sh -c 'echo -n "%: "; % -V'

/home/oli/venv/bin/python: Python 2.7.4

This is real output.

The fact is there could be hundreds of different versions of Python secreted around your system, either on paths that are contextually added, or living under different binary names (like python3).

If we assume that a Python binary is always going to be called python<something> and be a binary file, we can just search the entire system for files that match those criteria:

$ sudo find / -type f -executable -iname 'python*' -exec file -i '{}' ; | awk -F: '/x-executable; charset=binary/ {print $1}' | xargs readlink -f | sort -u | xargs -I % sh -c 'echo -n "%: "; % -V'

/home/oli/venv/bin/python: Python 2.7.4

/media/ned/websites/venvold/bin/python: Python 2.7.4

/srv/chroot/precise_i386/usr/bin/python2.7: Python 2.7.3

/srv/chroot/trusty_i386/usr/bin/python2.7: Python 2.7.6

/srv/chroot/trusty_i386/usr/bin/python3.4: Python 3.4.0

/srv/chroot/trusty_i386/usr/bin/python3.4m: Python 3.4.0

/usr/bin/python2.7: Python 2.7.6

/usr/bin/python2.7-dbg: Python 2.7.6

/usr/bin/python3.4: Python 3.4.0

/usr/bin/python3.4dm: Python 3.4.0

/usr/bin/python3.4m: Python 3.4.0

/web/venvold/bin/python: Python 2.7.4

It's obviously a pretty hideous command but this is again real output and it seems to have done a fairly thorough job.