How to discover what Linux distribution is in use

cat /etc/*release

Most distributions put a release file in /etc/ (like /etc/redhat-release, /etc/gentoo-release, etc.) which usually has the version number of your distribution in it.

lsb_release -i may work for you.

More detail is available with lsb_release -a

Some discussion at http://etbe.coker.com.au/2007/08/30/identifying-the-distribution-of-a-linux-system/