Detecting the presence of wifi devices
You will need a wireless nic that permits a 'monitor' mode, and that requires some special drivers and special nics.
- Kismet's list of cards for Linux
- tests on different cards
- pineapple device (special device)
- airpcap nic (specialized nic for Windows)
Once you get a card, you will need software to deal with the data and Kismet is the leader in this area right now.