Originally posted here by gore
Havn't read all of this because I'm busy, but for this one. Yes, with a ping sweep.
i think he meant with a nat on the router, any ways if you have a NAT isntalled on the router then it will be much difficult to detect those PCs, if you have a firewall configured right then it is almost imposible.
any ways just download Nmap from www.insecure.org and then run it as such
nmap -sP [ip_address]

check man nmap for more details.