Fedora Core: /etc/fedora-release
Red Hat: /etc/redhat-release, /etc/redhat_version (rare)
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.