Hi,
The only advise I can give you, is to use make a capture of traffic to and from the Citrix Server (use pass-through if you have this available), with a filter on IP or MAC of this server. If you then open a session and work on a terminal connected to the terminal server, you should be able to detect the RDP or ICA protocol(s). You can then set a filter based on the frames you detected.
Regards,
C152