I am assuming you want to see if you have wireless freeloaders on your
network? You should be able to see this information using the software
built in to your router. Your router has an IP address just like your other
networked machines, usually 192.168.1.1 or 192.168.0.1, just type that
address into a web browser and you should see some kind of web page asking
for a username and password, or showing you configuration options. The
first thing I would suggest you do is put a strong username/password
combination in place on your router. This way if someone does get access to
your network, they can't start changing your router's configuration to suit
their needs. Once you're into the router's interface (each brand has their
own software), browse around a bit and you should be able to find out who is
connected.