sudo iwlist wlan0 scan

You'll see output resembling:

Cell 01 - Address: 00:11:22:33:44:55
      ESSID:"network-essid"
      Mode:Master
      Channel:11
      Frequency:2.462 GHz (Channel 11)
      Quality=100/100  Signal level:-47dBm  Noise level=-100dBm
      Encryption key:off
      .
      .