How to get the signal strength of the network connected with the ESP8266
The page http://tomeko.net/other/ESP8266/ names the following command:


  AT+CWLAP     // list available access points


According to the example given there, this returns a list of items looking like this:


  +CWLAP:(3,"UPC784xxx",-75,"70:54:d2:xx:xx:xx",1)


where "APs are sorted by channel and values are: (security, "name", signal strength, MAC, channel) where for security: 0 = open, 1 = WEP, 3 = WPA, 4 = WPA2".

Thus you can use AT+CWJAP? to obtain the name of the network you're connected to and get the signal strength from the respective item in the list provided by AT+CWLAP.

See http://wiki.iteadstudio.com/ESP8266_Serial_WIFI_Module for a reference of commands.