How does Mac OS X 10.7 detect different displays?
1) You should be able to see these configurations in System Preferences > Display. I believe you go to to Layout (or could be called mirroring).

2) OS X displays two Display panels so you know which panel is attached to which screen.

If the Display options don't allow you to pick a resolution, change the Resolution from "Best for XX display", that will allow you to pick a supported, lower, resolution.