You can see this from the new topology view (at least if you have a switch in the network). Below you can see that port 3 on the MX is connected to port 8 on the switch. Note that this is LAN port 3 on this MX75 which is actually labelled 7 on the unit as they start from 0 and the first three ports are WAN ports.
If my answer solves your problem please click Accept as Solution so others can benefit from it.