How to check your CentOS version

Easiest way is to use command line.. 

cat /etc/centos-release
Or
cat /etc/redhat-release

CentOS version history follows that of Red Hat
If you want to know more details on your kernel etc.. use the below command: 
uname -a
Sample Output
Linux sample.com 2.6.18-194.26.1.el5xen #1 SMP Tue Nov 9 13:35:30 EST 2010 x86_64 x86_64 x86_64 GNU/Linux

There you could be newer kernels installed on your CentOS server but not actually running. You can list the installed kernels using yum or rpm. You can then see which kernel is set to start on boot by checking your /etc/grub.conf.

Another great way to get all this information would be to use lsb-release..: 


lsb_release -a
LSB Version: :core-3.1-amd64:core-3.1-noarch:graphics-3.1-amd64:graphics-3.1-noarch
Distributor ID: CentOS
Description: CentOS release 5.5 (Final)
Release: 5.5
Codename: Final