You need to know the MAC address of each of your access points - also known as the BSSID.
It should be available in the admin UI of the access point, and sometimes also printed on a label attached to the AP.
Knowing that, you need something that shows you details about wifi connections, so you can view the BSSID (MAC address of the AP) you're connected to:
Android, iOS: Install the "Network Info II" app
Linux: use the iwconfig command
Windows: Use the command "netsh wlan show interfaces"
