It is very difficult to remember the release version for each of unix servers if you have a lot of different Linux distributions installed on many different machines. But, no worry as there is easy command that can help you to trace the release version of the unix operating system.
If you are on RHEL/CentOS
[root@server ~]# cat /etc/redhat-release CentOS release 5.5 (Final)
If you are on Sun Solaris
If you are on SUSE Linux
If you are on Ubuntu Linux