you can also go to the operating company's website, and type in your departure/ arrival cities and exact same time. That should pop up the operating carrier's flight number. If it's a star-alliance flight, it can be easily done on their website, which lists all the flights by different star carriers in and out of a city pair for a day. You should easily spot the code share flights and operating carriers' flight numbers, as they arrange the list by time.

http://www.staralliance.com