You could use the RFT Test Command. You would need to have a WLAN client very close by for the AP to send some frames to:
rft test profile antenna-connectivity ap-name <name of ap> dest-mac <mac of client nearby> radio <0 or 1 depending on what band the client is on>
It would spit back out a transaction ID and then you would type "show rft result trans-id <transaction id>" to obtain the result. Below is a sample:
(Aruba7005-US) #rft test profile antenna-connectivity ap-name Office-325 dest-mac 64:30:99:92:4a:5f radio 0
Transaction ID: 801
(Aruba7005-US) #show rft result trans-id 801
Profile AntennaConnectivity, TransID 801, AP 192.168.1.102, Dest 64:80:99:92:4a:5f, Radio 0
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Antenna Connectivity Test Result
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Antenna 1: Avg S/N ratio: 0 Success Rate: 99%
Antenna 2: Avg S/N ratio: 0 Success Rate: 100%
Difference: 0 1%