The easiest way on how to check what Debian version you are running is to simply read a content of /etc/issue file. Example:
root@debian:~# cat /etc/issue
Debian GNU/Linux 7 \n \l
However, the above command may not show the current Debian update point releases. Thus you may get more accurate info with the following command:
root@debian:~# cat /etc/debian_version

An alternative way is by use of lsb_release command. This command may be missing by default so you may need to install it first:
# apt-get install lsb-release
Once you install the above package run the following command to see what debian version you are running on your debian box:
# lsb_release -da
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 7.7 (wheezy)
Release:	7.7
Codename:	wheezy
For systemd debian version you may also use hostnamectl:
# hostnamectl 
   Static hostname: debian
         Icon name: computer-vm
           Chassis: vm
        Machine ID: 4c983dae19f6403ca2c555c3867beb14
           Boot ID: 3b399eb976d04569a8514a1d666a4d81
    Virtualization: oracle
  Operating System: Debian GNU/Linux 8 (jessie)
            Kernel: Linux 3.16.0-4-amd64
      Architecture: x86-64