Re: Who's goes there?
if you have a shell at the target, you could try a series of tools, like netstat, ifconfig, etc, to find out who/what else is connected.
You would still be limited by your access permissions.
Nevertheless, the legality of these actions will depend on what has been agreed between you (or the company you theoretically represent) and the owners/operators of the target box. YMMV widely, since some people out there are very paranoid.
Hoc volo, sic jubeo, sit pro rationa volontas.