First, the wireless interface has to be activated. In this case I am using Debian and the interface name is wlan0.

Step 1. Activate the wireless interface

ifconfig wlan0 up

Step 2. Scan for the wireless networks

iwlist wlan0 scan

The output should be sometshig like this:

wlan0     Scan completed :
          Cell 01 - Address: 00:17:53:0B:30:C0
                    ESSID:"linksys"
                    Mode:Master
                    Channel:6
                    Frequency:2.437 GHz (Channel 6)
                    Quality=47/100  Signal level:-76 dBm
                    Encryption key:on
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/s
                              12 Mb/s; 24 Mb/s; 36 Mb/s; 9 Mb/s; 18 Mb/s
                              48 Mb/s; 54 Mb/s
                    Extra:tsf=0000010b1bfe2181
                    Extra: Last beacon: 28ms ago