Who's on Your Wireless Network? Find Out!

"How to Tell If an Outside User Is on Your Wireless Network"
wikiHow

"Wireless security is very important these days. You don't want anyone stealing your bandwidth or getting into your network to perform malicious attacks on your computer. This is a guide to help you know when someone else is on your wireless network. Since every wireless router is different, this article will discuss the basics and use one of the most popular wireless routers, the Linksys WAP54G as an example...."

Looks like a good how-2 - and a potentially very useful one.