How do you see what other people are doing on your WiFi network?

Depending on what firmware is on your router you can check out the log files. Though the most you are going to get from this a MAC address, which really isn't much good for finding the person. The only real upside is you can block their MAC and most attackers seem to have a hard time to figure out why they cant get back on all of a sudden. Usually I just put a passWord on the actual router but leave wep/wpa key off. Set-up Vlan. Set-up QoS. Give myself the best stream and set everybody else to a setting that makes dial-up look fast. Most people give up when they see the terrible speeds they are getting and wont bother the network again. If you want to be nasty, let them connect and sniff their traffic. Next time they go online they will see their emails are all deleted except one which pretty much warns about the dangers off connecting to open networks.