All you have to do is Diagnostics > Packet Capture on WAN for port TCP 445 then run a scan.
If you get a connection refused (CLOSED) but do not see the traffic on WAN, then something upstream is responding.
If they are responding AND forwarding the traffic to you (which wouldn't make much sense) then you will see the SYN to port 445 on your WAN but no SYN/ACK response because you are blocking the port.