Oh, I see what you mean. You need to know the actual flight number on the other airline. Well, I am not aware of any "list" of such thing. You need to look up the timetable of the other airline and compare.

Or, on some flights, both airlines will have flights show up on results of an itasoftware.com search. You can cross-check the flight numbers there.

But in short, no easy way to find out a lot of flight numbers on a lot of codeshare partners.