Fedora Core: /etc/fedora-release

Red Hat: /etc/redhat-release, /etc/redhat_version (rare)

via release-files.

I got confused between two Centos VMs.  I had thought one Centos VM that I have running DNS was Centos 5.4 and the new one was Centos 5.6.  When I saw that they both used a 2.6.18.* kernel I got confused.  Fedora Core is using 2.6.35 and greater.  This led me to check the distro version upon which I didn’t get it out of uname -a or dmesg.  Searching the intertubes and I got the above answer.  Apparently the VM I thought was 5.4 is really 5.5.

It is interesting that Centos, which tracks RHEL, doesn’t make too many radical changes to the kernel.

Update: It should be noted that I wouldn’t have noticed the kernel versions had I not tried to compile and install my own kernel.