Originally Posted by
DY444
I'm not a ticket expert but what seemed to work for me was checking if the route was bookable on the desired codeshare airline's website. So for example if you want to fly from A to B with an AA flight number then see if you can book the desired flight on the AA website with AA codeshare flight numbers. If not you most likely can't book it because if the marketing airline doesn't let you book it then who will?
The flight being available standalone is a sufficient but not a necessary condition. For example AA have been happy to book me on AA, AY, and JL codeshares (sometimes even despite traffic restrictions) even though none of those flights would be bookable on aa.com.