There are a number of ways you can narrow down which plane you would be on. I know AA lists the aircraft if you look at the details of your flight. You should also be able to narrow it down by what aircraft a carrier operates. For example (until recently), AA didn't have any 777-300's so if you were flying AA on a 777, it had to be a 777-200. You can also look at the seatmap and match up defining features, such as the number of rows or what rows are marked as exit rows.