Recently (this morning) I had to find out which version of Ubuntu was running on a clients machine.  A quick google provided three articles of note all with different solutions.

  1. Type `cat /etc/issue` into the command line (no back ticks).
  2. Type `lsb_release -a` into the command line (no back ticks).
  3. If you have a GUI, open up FireFox and view the about dialog.