Sniffing other user's session

Hi people! We have here a server and a couple of X-terminals connected to it (X over ip). Is it somehow possible for a regular user to set a sniffer or something similar (as user of the server on the server) so that he/she can "see" the whole X-session another ones who sits at a terminal? Should I consider tunneling all X-sessions through ssh, or is this unnecessary? Thanks a lot in advance! JLT