How to find out if someone's tapping my wireless network?

Yes there is software that you can install to check, but it'll be easier to go to your router settings and check the DHCP client list and see if others are using it. Enableing WPA/WEP security is your best bet to stop others from using your net. For the software to check if others are using your wifi it is called wireshark. You will need to download it from www.wireshark.org. The software is meant for system admins and network admins but a regular user should beable to figgure it out. If you have any other questions about how to use wireshark just contact me.

Firstly, make sure you have encryption on. WEP will offer basic protection against casual visitors, but isn't very secure. Aim for WPA if possible. Also, might be worth using some kind of MAC address filter. In terms of checking for taps, you could use a separate device (eg. a laptop with a wireless card) to check what other wireless devices are in the area and log this somewhere. Your wireless router might also be able to log connection attempts etc so that you can see who connected and when. Unfortunately, as with anything in IT, a determined hacker will generally find a way. Remember, security is a process.