No external device needed, just root access and I usually make a symlink to somewhere on $PATH:

  $ sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport  ~/bin/airport
  $ sudo airport sniff
  $ tcpdump -r /tmp/airportSniffxxxx.cap | less

You can even use the GUI: http://osxdaily.com/2015/04/23/sniff-packet-capture-packet-t...

There's bunch of cool options, including WiFi quality scan.

It sure does, which is awesome! The UI even has support, and when it’s in monitor mode the WiFi status icon even turns into a solid fan shape with a little eye in it. It’s the little things.

‘jzelinskie has the short instructions in their comment.