While not the best option, you could do a show port/show interface (CatOS/CatIOS) on each port/interface. If no traffic stats are shown on the port since the last switch reset/reboot, then perhaps that port is not in use. Of course if your switch rebooted recently, this might not be valid.
You always have the option of shutting down all ports that are disconnected, then waiting for the phone calls from irate users. :-)
HTH
Steve
