Whenever you buy a ticket, there's (IIRC) Marketing or Plating Carrier, that's who SELLS the ticket.

It can be another carrier, usually in same Alliance (oneworld in AA's case) whose plane (or as we call it here, metal) is actually used for the flight.

My experience is with UA and * Alliance, and I've seen Thai flight prices quadruple (even more) on some flights if I want to purchase it as a UA coded flight.

But I think this may be going a little OT. If you want info on AA codeshares or using giftcards to buy tickets, I'd suggest you ask in AA forum.

One other though: I know for a fact that some places in Canada can be VERY expensive to get to, but generally speaking, using miles from US to Canada is NOT the best value for said miles.

OK, I may not have actually aswered the question itself. So, if buying online, look to see if flight number starts with AA, eg AA 1102 (to pick a random flight).