Have a read up on it

http://www.netstumbler.com/

Basically its a small program that you leave running whilst you drive about, as you drive (providing the refresh rate is set correctly) it shows networks in range, the signal strengh, and when they go out of range.

It also logs the wireless networks on a map if you got GPS on your laptop, so you can build a database of where networks are

Its better if you use an omni directional aeriel, because when you point it towards the AP the signal shows better, when you point it away from it, the signal drops. Its to give you a good idea of where the AP is