While it's not an actual list, if you go to:
American Airlines Map you can click on Destination, enter the airport you want to leave from and tick the box Add AA partners. At the bottom of the screen, it will display a list of flights and anything that is a codeshare will say operated by XXXX
Once you do that in the lower left hand corner there is a box, and you can select Codeshares in the widget. The lines will change color to show if the route is operated by AA, there are some codeshares on the route, or the route is all code shares.