How to check the Debian version
Command cat /etc/issue
The easiest way to find out your Debian version is to look at the contents of the /etc/issue file with the cat command
root@debian-host:~# cat /etc/issue
Debian GNU/Linux 8 \n \l

Command cat /etc/debian_version
However, this file contains only information about the version of the distribution, in order to find out the version and release number of Debian, you need to use another command:
root@debian-host:~# cat /etc/debian_version

Program hostnamectl
Starting with version 8, it became possible to use the hostnamectl command, which will display even more information about the installed system:
root@debian-host:~# hostnamectl
Static hostname: debian-host
Icon name: computer-desktop
Chassis: desktop
Machine ID: 21ea4bf892e94d309288ef6284bff8e0
Boot ID: b8d0d1cdb10348f2bbbb59d07da5713f
Operating System: Debian GNU/Linux 8 (jessie)
Kernel: Linux 3.16.0-4-amd64
Architecture: x86-64

Program lsb-release
In addition, you can install the lsb-release utility, which is not available by default, and use it to find out the version of the operating system. To install, just use the command apt-get install lsb-release, and then run this command:
root@debian-host:~# lsb_release -da
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 8.7 (jessie)
Release: 8.7
Codename: jessie