CentOS VNC Xen and X Server

[Velocity]

I do not have physical access to a CentOS server I have access only
via SSH and it does not have GUI I have only been acustomed to GUIs
though I have used ssh in past I installed GNOME Desktop Environment
by yum groupinstall "X Window System" "GNOME Desktop Environment"
but then I am not clear wether it is working or not I tried to access
it using vnc but then I got errors that unable to open display so I am
wondering if X is working on the system or not.Since I do not have
physical access so I am not clear with the process.

I added a user for accessing vncsessions edited /etc/sysconfig/
vncservers

and in xstartup file of the user in his .vnc directory
did changes as mentioned here http://wiki.centos.org/HowTos/VNC-Server
[vncusers@testmachine root]$ vncviewer localhost:1

VNC Viewer Free Edition 4.1.2 for X - built Mar 24 2009 19:52:30
Copyright (C) 2002-2005 RealVNC Ltd.
See http://www.realvnc.com for information on VNC.
vncviewer: unable to open display ""

I did tried ssh -X <remote host> and run xclock
it did not worked it gave following error
Error: Can't open display: