If your budget for wireless network detection tools is up to and including $60, then I think I can safely say that Wi-Fi Networking News would recommend you buy the Canary Wireless Digital Hotspotter Model HS-20.
It’ll be available in about a week and includes 802.11n network detection, the speeds of various available networks, network names, types of encryption, network channels, and more. Check out the above video for a quick demonstration of the device.
The Latest, Greatest Handheld Wi-Fi Detector [Wi-Fi Networking News]