Unfortunately, there is no obvious way to do it. You'll have to check /etc/config/mwan3 config for metric settings. Lower the metric - the more priority interface has. So let's say you have failover set-up and one of the interfacess are down - failover will regroup them so that active network had a higher priority in that config.
Another way to do it would be pinging the WAN IP of all the interfaces and see which one responds, this way you will be able to detect which one of them is active.
You can use ifconfig command too, to see current traffic, detailed info about the interface and so on.
One more thing to check - syslog, run logread command to open system log and see what were the last events and what switched to what.