How to tell if the iphone 5 is connected to the 2.4 or 5 ghz network?
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Since you are using an AirPort Extreme, you can set up a separate SSID for the 5GhZ-network and connect to that one manually on your phone, and from there be able to see which network you're on. To do this, follow the below step:


  Airport Utility 6.0: Airport Extreme > Edit > Wireless > Wireless Options > Tick the box next to "5GHz network name".
  Airport Utility 5.6 and earlier: Manual Setup > Wireless > Wireless Network Options > Tick the box.
  Source


P.S. Having a different SSID does not make it a separate network, it just makes the two bands differentiable.