Most Red Hat-based distributions, like CentOS, should have a file called redhat-release which will contain the CentOS version
Run the following to see what version running on your system
tail /etc/redhat-release
or
cat /etc/redhat-release
or
cat /etc/*release*
or
rpm -q centos-release
Using your redhat-release file, you can find your distribution, version and update number. This information is very helpful for technical support staff as well as knowing if you are up to date with the latest patches