How to tell if the MBA (Mavericks) is connected to the 2.4 or 5 ghz network?
Option + Wifi Menu

Hold the Option key down and select your WiFi icon in the menu bar ( &#x2325; +  ).

Additional information in the menu includes “Channel:” and (5 GHz) or (2.4 GHz) for the current radio settings.

As seen in Yosemite:



Wireless Diagnostics.app

You can get even more information from System Information. But I usually open the Wireless Diagnostics app (in Applications > Utilities folder). Choose Window > Info to get more detailed WiFi networking or scan for better channels, check for interference, etc.