The feature is missing from the WebUI at the moment. From 7.4 firmware release, WebUI Overview window should have indication of currently used Main WAN interface.
As of now, you can check Network -> Interfaces section, where Main WAN is the highest online interface in the list.
It can also be derived from the routing table from Status -> Routes -> General routes section as a route with lowest metric.
A couple of CLI/SSH commands:
- ubus call vuci.network.network interfaces_status | grep Main
- ip route get 188.8.131.52
will also provide you main WAN interface name.