Originally Posted by CApreppie
Definitely check-in with the airline whose plane you are flying. Codeshares are tricky. I think they mess up a lot of travelers who end up going to the wrong counter first and then have to trek to the right one.
Good luck with that! I had an AS codeshare on AA equipment. AS TAs were not scheduled to be there for several hours and AA kiosks would not recognize me. The Platinum line helped me out. But in general I agree. Go to the equipment carrier first as they are often in different terminals. They should be able to check you in!