Try this:
Step 1: unplug the cable going to the Port 1 connector on the InterLink.
Step 2: power both units on and go to the System Status screen to see what accessories each unit is showing as connected. The unit that shows the InterLink as connected would be the one still plugged into the Port 2 connection on the InterLink. The unit not showing the InterLink as connected would be the one connected to the Port 1 connection on the InterLink.