If you plug your flight number in at
FlightAware.com there's a link (toward the top on your phone, or on the right on your computer) that says either "where's my plane" or "track inbound flight" and you can find out exactly where the flight originates from. Note that this may differ on different travel days, but the site lets you go back or forward a few days to find your exact flight (or the most recent one on the corresponding day of the week). This time of year, I always track both my flight and my inbound.