There are a few ways to check what version of Debian or Debian-based Linux (Ubuntu, etc) that you are using.
This file may not be available.
Output will look like this
root@unknown_server:~# cat /etc/debian_version 7.5
root@unknown_server:~# cat /etc/issue Debian GNU/Linux 7 \n \l
root@unknown_server:~# lsb_release -a No LSB modules are available. Distributor ID: Debian Description: Debian GNU/Linux 7.5 (wheezy) Release: 7.5 Codename: wheezy