I and some other people that I work with, use a program called
Look@LAN for trying to find out if an IP address is/was in use.

Basically, it's better to us, than a ping because a ping only tells
you if it's currently on, yet Look@LAN, will usually show offline but
telling you that the IP is in use, just not at that moment, so we can
avoid "later-on" collisions.

Though, we had an IP conflict once, where I guess the firewall was on
and we tried the software and it claimed the IP isn't nor was in use,
but Windows XP noticed it had an IP conflict.

How does Windows XP know this? We figured probably NetBIOS back to IP
or something and we would be curious for a program that perhaps was
more reliable to use, and could use whatever method Windows used to
find out that the IP was actually in use, and not cause us problems.