How can I see traffic from shared Wifi (Macbook AirPort)?
tcpdump is a command-line utility to see the traffic on any network interface.  It is full-featured, but can require some serious reading of the man page.

Wireshark is a full-featured application that will allow you to see all such traffic.  It provides a GUI (X-windows based) into which you can specify filters on traffic and collect statistics.  See more info and download it at wireshark.org.  I find knowledge of Wireshark to be very, very helpful with lots of network tasks.