How can I tell when a Mac switches access points?
Hold down the option key and click on the wireless icon on the menu bar. It should show you the SSID, channel, and Hardware address of the access point you are connected to. Source
I got some mileage out of doing this:
while true ; do /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep BSSID ; sleep 1 ; done
or
while true ; do clear ; /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I ; sleep 1 ; done