CANARY WIFI DETECTOR (Canary Wireless HS-20 Digital Hotspotter) is the best way to find and count WIFI networks around you.
Because of the signal structure of WIFI and the fact the signal is still usable down to -102dBm (and the most sensitive RF meter I know, the CORNET ED88T goes down only to -60dBm= 0.0005mW/m2) RF meters can "see" WIFI routers "only" up to 10 meters from the router.
So in order to cover this, and to know if, how many and how strong the WIFI signals are around the house I am going to rent or but, I use the CANARY WIFI DETECTOR .
In contrary to a smart phone, it does not emit RF radiation while searching for the signals, making it a very good tool for EHS people. But on the other hand, smart phones will probably be more sensitive then it.
Last time I checked it cost start from $58.99
You can order it from Amazon.com: