Detect Apple Remote Desktop (ARD) client at server?
to block: you could have your switches filter ports.  though, you might also block vnc if you do 5900.

or to detect, i'd say use something like bonjour browser utility to find services, or nmap to find active ports.  

3283    TCP/UDP     Net Assistant   -   net-assistant   Apple Remote Desktop 2.0 or later (Reporting feature)

5900    TCP     Virtual Network Computing (VNC)
(Unregistered Use)  -   vnc-server  Apple Remote Desktop 2.0 or later (Observe/Control feature)
Screen Sharing (Mac OS X 10.5 or later)