If you have an Ubuntu installation, and want to check what version it is, you have a number of options. The most "right" way I've found is to simply look at the /etc/lsb-release file, which should tell you explicitly what version of Ubuntu is installed, its codename, and its version number.
Or, if you like the command line, run lsb_release -a
Of course, you could do a number of other things instead, like looking at /etc/apt/sources.list to see what version of Ubuntu you're pulling packages from. But I thought the above method was nice, and not often mentioned.