No way to get this info online: it's in DECS, which is an operations system that AA does not expose to the outside. At times the GA will put the flight number in the comments for the delayed departure time, so you'll find it in Sabre abd therefore in expertflyer.

However, if you're at the airport, it's just as easy to scan the "Arrivals" monitors for a flight that gets into the same gate you're departing from.