Aruba APs were not meant to have static channels, so it can be done, but it takes a little work:
To manually set a DFS channel for an AP, first create a regulatory domain profile removing the regular channels then adding a single DFS channel, then configure it, using ap-specific: Here is an example using channel 60:
ap regulatory-domain-profile "Channel60"
no valid-11a-channel 36
no valid-11a-channel 40
no valid-11a-channel 48
no valid-11a-channel 153
no valid-11a-channel 157
no valid-11a-channel 161
no valid-11a-channel 165
valid-11a-channel 60
!
ap-name "Kitchen-225"
regulatory-domain-profile "Channel60"
If you apply that to an access point with ARM, it will only select the only channel in its regulatory domain. In the specific configuration above, this will only work if you have the AP configured for 20 mhz channels. You would have to choose a regulatory domain profile with 40mhz channel pairs if you want to test at 40mhz.
To see if the AP has had any DFS events, I would look for radar messages in the wireless log:
show log wireless 50
Aug 27 10:21:23 :404076: <WARN> |AP Kitchen-225@10.99.154.29 sapd| AM 18:64:72:fb:30:50: Radar detected on interface wifi0, channel 132, typeid 7
Aug 27 10:21:24 :404088: <WARN> |AP Kitchen-225@10.99.154.29 sapd| AM 18:64:72:fb:30:50: ARM HT Radar Detected Trigger Current Channel old 132E new 100E/6