I have three WiFi access points (one ca. 2010 Apple Airport Express, one previous-generation Apple Time Capsule, one ASUS RT-N66U) configured in bridge mode and connected by Ethernet to a current-generation Apple Time Capsule which acts as a router, and also as an access point in its own right. To facilitate roaming, all four devices share the same network name, security mode (WEP2 personal), and password.
How can I tell (ideally in OS X and/or iOS) which access point I'm actually connected to at any given time? E.g., how can I confirm that I'm connected to the nearby Airport Express rather then the RT-N66U two rooms away?