You could try /who <ip> or on an unrealircd server /who +h <ip>.
The majority of servers now enforce an automatic +i (invisible) on connect, meaning 'invisible' users not in any of your channels will not be searchable by /who.
If you have any security concerns, an IRC operator will (probably) be more than happy to tell you who and what is connecting under your host. You should ask in your network's #help channel.
Hope that helps