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 9 \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
Next, you can check for /etc/os-release release file:
# cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 9 (stretch)"
NAME="Debian GNU/Linux"
VERSION="9 (stretch)"

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 9.0 (stretch)
Release:        9.0
Codename:       stretch
For systemd debian version you may also use hostnamectl:
# hostnamectl 
   Static hostname: debian
         Icon name: computer-vm
           Chassis: vm
        Machine ID: 6866ec1d654b4a2ea52d591dc0146d82
           Boot ID: 51bfbc1e197b4d378e95073da0df0288
    Virtualization: xen
  Operating System: Debian GNU/Linux 9 (stretch)
            Kernel: Linux 4.9.0-1-amd64
      Architecture: x86-64