Check the airport's Website and see if you can match up the arrival gate and departure gates. Of course if you're on an RJ that pulls up to a gate that is used by more than 1 plane, that could be tough. For me at IND, this is largely the way I go about doing it.

Also, if you're signed in on FlightAware, I have noticed that there's now an option to track the incoming flight. This is largely accurate except for instances where DL subs a plane for a very late arriving plane which has happened to me previously.

Would be really nice if DL took a hint from CO's site re: flight status.