FlyerTalk Forums - Where does flight originate from

Check out the info listed in section 6.1.4 of the Wiki FAQ, linked from the top of this forum:
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The Flight Status information page at southwest.com is a treasure trove of routing information. For example, if you ask about the arrival of your flight number into your departure city, you can find out where the flight is coming from and whether it is leaving that city on time. Then you can ask for the arrival time of the same flight number into that city. This method allows you to work upstream all the way to the origin of the flight and make your own guess as to the downstream delay, if any, in your city. If the flight is originating at your city, you may be able to locate the flight scheduled to arrive at the same gate 30 minutes before your departure and check its status. Unfortunately, this method will not help you detect a late-arriving crew or a tail swap.
On http://www.flightaware.com you can enter Southwest and the flight number to see that flight's entire itinerary over the past few days. Note that the segments read upward; that is, the last segment is at the top. It takes some interpretation to see that a flight was diverted on a previous day. Note that Saturday schedules are completely different from other days.
You can also search http://www.flightarrivals.com . You can enter a flight number and see its full itinerary. If you want to know on an earlier day whether the flight is originating or continuing (which could influence your boarding strategy), you'll need to check this on a day when the flight follows the same schedule as on the day of travel, and while that flight is in progress or close to it. That is, if you're flying on a Saturday morning, you may need to search this on a Saturday morning.
http://www.flightstats.com has an iPod/iPhone-friendly interface. If you aren't taking a continuing flight, you can use flight status by airport to see what flight is arriving at the gate from which your flight departs, and see if that flight shows a delay.
If you're interested in seeing the intermediate stops for a future date, possibly after a schedule change, you can go to http://www.southwest.com/cgi-bin/requestSchedule (you can also get there from southwest.com, Travel Tools, Schedules), select Download Schedules for the city in question, then the date range for the date that interests you. You get all the flights to and from that airport; you can either look it over for the flight number, or do a search in the PDF reader for the flight number. You can also use this to determine if the flight is continuing from someplace else.
flightlookup.com redirects to http://www.eskyguide.com , another place to look up itineraries of flights on future dates.