There are a couple of ways to find the CentOS version from the command line. The most accurate way is to check the installed centos-release rpm another option would be to cat the contents of /etc/redhat-release (not recommended as any application could potentially change the contents of this file).
CentOS Version Command
The most accurate way to check the CentOS version:
A far less reliable way of checking would be:
Below is a video tutorial on how to find the CentOS version: