I'm not sure I understand the question entirely (is this a reverse proxy? I'm hung up on the notion of a server being the client I guess), but there is a list of public citrix server IP ranges in recent rulesets. Lists > Subscribed Lists > Add list > check list is remotely managed, and choose list content of "Citrix IP ranges" if that might be of any help.
This is not what I mean.
We have our own CITRIX Server in out intranet.
We like to use dedicated rules for users coming from one of this servers.
Unfortunatly we don't know all installations worldwide.
So is there a way to detect such servers maybe by using the html header?