There is no easy way to do that.
You could sort the syslog and filter the logs per public IP.
let's say WAN1 has public IP 1.1.1.1 and WAN2 public IP 2:2:2:2, in the syslog you will see the WAN IP.
You could also do a packet capture and see where the trafic is egressing.
Currently the only ways that I'm aware.