I'm pretty sure it is illegal, at least in some states- but I don't think it's considered theft. Finding out about it shouldn't be too hard...you'll need to enable logging, assuming your router supports it.

Read up on wireless security- never ever leave the default settings on, as they are such that any computer with a wireless ethernet adapter can connect. You need to put in place MAC address filtering and WPA encryption. They are a 99% sure-fire way to deter most people looking to get on your network.

As for how to set this up, it will depend on your router, so look in the manual for more information. If the cards or router don't support WPA, use WEP as it is the next best thing (though certainly not the best option- MAC filtering will help remedy this).

 

Thanks, Fox. Is there anyway to tell who it is if I get their IP address?

 

The most you'll get out of any log is a MAC address, which can't really be traced back to anyone by any conventional means.

 

Sounds like you're concerned about "wardriving" a google search comes up with lots of stuff. Look here for example. http://www.wardrive.net/

 

In my netgear router setup program there is a screen that shows who is/has been attached to my router and the mac address.

If you setup the wireless security on your router, it is much less likely someone is attached to yours. There are to many open access points in most cases for someone to try to break into a secure one.