If you want to go beyond just detecting the existence of a wireless network and actually inspect the contents of the wireless packets, you will need a wireless sniffer. Similar to Ethernet sniffers (sometimes called protocol analyzers), these applications can show you the contents of the packet down to the bit level. This is very useful for understanding traffic flows and performing basic troubleshooting because you can see what's going on.
There are 2 commercial wireless sniffer packages available:
AiroPeek, by WildPackets. (http://www.wildpackets.com)
Sniffer Wireless, by Network Associates. (http://www.sniffer.com)
If you don't have the budget for a commercial package, you could also use freeware called Ethereal (http://www.ethereal.com ...