International Co-share flights, Who do you check in with?

Here's a tip: when you see or hear the words, "Operated by..." that means you have to check in on that operating carrier. Example: Delta Airlines operated by Northwest. You must check in at the Northwest counter. You ticket means, you paid Delta and it's validated on Delta but the actual plane that will bring you to your destination will be Northwest. In the airline/travel agency world, we call this, code-shared flights:) Hope this helps:)