Check your About This Mac from the black Apple icon in the upper left hand corner. Select Overview and go down to  Network/WiFi. 
This is what my Mac Mini supports:
Interfaces:
en1:
  Card Type:   AirPort Extreme  (0x14E4, 0x10E)
  Firmware Version:   Broadcom BCM43xx 1.0 (7.15.124.12.10)
  MAC Address:   
  Locale:   FCC
  Country Code:   X0 
 Supported PHY Modes:   802.11 a/b/g/nI don't think AC mode is supported.  
