How to see who is using my WiFi

Usually a WiFi router has a web interface that shows all (previously) connected WiFi devices... Try going to the IP address of your router through your browser.

Read this part, and the rest of my blog post about WiFi security here: http://blog.superuser.com/2011/03/28/iron-cladding-your-wi-fi-network/

Happy reading!

first, it helps to understand how WIFI works, read http://www.wi-fiplanet.com/tutorials/article.php/1447501 and/or http://www.cs.wright.edu/~pmateti/InternetSecurity/Lectures/WirelessHacks/Mateti-WirelessHacks.htm.

any other machine "using your network" has to be associated to your network which means it sends "associated beacons" (as explained here).

then all you have to do is to grab a tool which sniffs WIFI-packets and then analyze, which of the received packets are meant to be used "in your network", you can find a list of tools over at http://www.topbits.com/wi-fi-software-tools.html.

and then you can compare the MAC address of the sender with the MAC address of the devices you allow and hope, that the sender did not fake its MAC address :)

If you're worried, changing the password on a regular basis will ensure that if somebody does break in, they won't stay for long.

Obligatory XKCD reference: http://www.xkcd.com/341/

as bloodphilia has said, you could find this you thorough the web interface of your point (typically 192.168.2.1 or 192.168.2.2, not always though). Mine at home will show the ip, mac and computer name so if there's a computer (or iphone, laptop, etc) you don't recognise you can set up a MAC Blacklist...or change you wireless key.