How can I use my Mac to determine the optimal WiFi channel to use with my wireless router?
You can easily determine the best channels to use in the native Wireless Diagnostics scanner:


Open the "Wireless Diagnostics" Application
Ignore the prompts and open "Window", "Scan" in the menu bar
Press "Scan Now"


The recommended channel(s) for both 2.4 GHz and 5 GHz will now be present in the Summary pane.



More info on OSXDaily.