Terminal at FCO

I think that was the old procedure. Then there was the fire and international checkin was moved. Not sure what the procedure is now and that makes this response nearly worthless. Just follow the signs.

When you arrive at the airport, look for the display boards that tell you where to check in for your flight. If arriving by train, the displays will be as you exit the train station. If arriving by taxi, look for displays inside the terminal.

Edit: The displays near the train station tell you the check-in terminal. The displays in the terminal tell you the check-in counter number. Note that it's a long walk from the check-in counters to Terminal 3 where flights to US depart, so allow time for the walk.

Yes, terminal 5 is closed for now, so you won't want to go there. It will be terminal 3 or 1. As previous poster noted, look at signs once you arrive at FCO. (Of course, it would be nice if you could find this out on United's website, but I just checked, and under "Check in Terminal" it's just blank. Very helpful.)

By playing around on the official (but not particularly user-friendly) Fiumicino website, I found that United flight 43 departs FCO terminal 3 on May 11, 2019 at 9:45 AM.

Thanks everyone. We will head to terminal 3 and look for the signs. P