How can I measure the WiFi duty cycle / channel utilization in MikroTik routerboard RB922UAGS-5HPacD using Winbox?
Duct tape it to a spectrum analyzer?
Wireless snooper will provide this information but it depends on how you have your system configured: http://forum.mikrotik.com/t/no-more-wireless-tools-snooper-sniff-spectral-scan-and-spectral-history/165372/1
It may not be possible to serve clients, either, so either a dedicated Mikrotik configured for this or, as others have said, move to a third party product.