Try lsb_release -a
guiverc@755-deb:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux buster/sid
Release: testing
Codename: buster
You'll note my debian is a 'testing' release, and thus not stable. Debian 9 (stretch) is the current stable release of debian.
The current stable distribution of Debian is version 9, codenamed
stretch. It was initially released as version 9 on June 17th, 2017 and
its latest update, version 9.8, was released on February 16th, 2019.
https://www.debian.org/releases/
On another box I get
guiverc@de2900:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 8.11 (jessie)
Release: 8.11
Codename: jessie
The 8.11 shows it's a stable release (could also be called an old-stable).
cat /etc/os-releases
? – Biswapriyo Mar 2 at 8:36