Most of our customers' VPS's run on CentOS 5.
To get your OS, if you are on CentOS you can look at the file /etc/redhat-release, by executing the following command:
$ cat /etc/redhat-release
This should output something like:
CentOS release 5.x (Final)
Where x is the minor version number.
If you don't have this file in your VPS, then you are running another type of OS like Ubuntu.