1. Open Wireless Diagnostics located in /System/Library/CoreServices/Applications/
2. Press ⌘Command+6 or choose Window > Packet Sniffer
3. Choose a channel and frequency
4. Click Start
Once you click Stop, a
.wcap file will be added to your Desktop. You will need a program like Wireshark to read the file. Wireshark can also capture packets, but it is interesting that Apple has added this function to Wireless Diagnostics.