And if you don't know what is the native resolution of your monitor, most of the time it's the last one (or highest one) on the list.
Correct me if I'm wrong, but usually with Windows when you don't use the native resolution you'll get a pop-up warning shortly after you log in. Some monitors also display a warning when outside the recommended resolution. But I think you get these warnings with DVI only, I don't think it works when connected with a VGA cable.