arping

Arping is a util to find out if a specific IP address on the LAN is 'taken'
and what MAC address owns it. Sure, you *could* just use 'ping' to find out if
it's taken and even if the computer blocks ping (and everything else) you still
get an entry in your arp cache. But what if you aren't on a routable net? Then
you're screwed. Or you use arping.