Below are some tools that will assist in checking wifi strength.
WifiInfoView by NirSoft
"WifiInfoView scans the wireless networks in your area and displays extensive information about them, including: Network Name (SSID), MAC Address, ...."
"Wireshark is a free and open-source packet analyzer. It is used for network troubleshooting, analysis, software and communications protocol development, ...."
Wifi Analyzer for Android
The advantage of having such smartphone app is that it is easier to carry the smartphone around the premises and analyse the wifi signal strength instead of carrying the laptop around.
Wifi Solver FDTD
$0.99 to buy (as of 17 Aug 2015)
This app is supposed to be based on some seriously advanced computational solver on electromagnetics signal. The downside is that the app is not free.