You can get detailed information about a wireless interface by using the iwlist command.

Run the below command from the command line, just copy and paste:

sudo iwlist wlan0 scan | grep Frequency | sort | uniq -c | sort -n

And you will get a result like this:

  2                     Frequency:2.412 GHz (Channel 1)

  2                     Frequency:2.417 GHz (Channel 2)

  2                     Frequency:2.462 GHz (Channel 11)

 10                     Frequency:2.437 GHz (Channel 6)

As an extra bit of useful data, you can see what frequencies your wifi card supports using this command:

iwlist wlan0 channel