How to check your CentOS version

Most Red Hat-based distributions, like CentOS, should have a file called redhat-release which will contain the CentOS version.

# cat /etc/redhat-release
CentOS release 5.5 (Final)

or

# rpm -q centos-release
centos-release-5-5.el5.centos.1

and finally

# lsb_release -i
Distributor ID: CentOS
# lsb_release -r
Release: 5.5
# lsb_release -d
Description: CentOS release 5.5 (Final)