You can use the following commands to check:
# lsb_release -a
LSB Version: :core-3.1-ia32:core-3.1-noarch:graphics-3.1-ia32:graphics-3.1-noarch
Distributor ID: CentOS
Description: CentOS release 5.4 (Final)
Release: 5.4
Codename: Final
This command works for all Linux distributions, including Redhat, SuSE, Debian, etc.
root@MyMail ~ # uname
Linux
root@MyMail ~ # uname -r
2.6.18-164.el5
root@MyMail ~ # cat /etc/redhat-release
CentOS release 5.4 (Final)
Log into Linux and run rpm -q redhat-release
#rpm -q redhat-release
or CentOS
root@MyMail ~ # rpm -q centos-release
centos-release-5-4.el5.centos.1