Originally Posted by BarbiJKM

Check the flight status on the SW home page, but put your departing city in the "arrival" box, with the fight number, and leave the departure city blank. If your flight is a continuation from another city, it will show up; if your flight originates in your departure city, you will get an error message. You can also backtrack the originating city the same way, to see if the flight came from somewhere else to there.