$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux testing (squeeze)
$ cat /etc/debian_version
Q. Ok, but how do I know which distribution I’m running?
A. If you are running testing or unstable, then /etc/debian_version is
not a reliable way to know that anymore. Looking at the contents of
your /etc/apt/sources.list file is probably a much better way.