CRT multiscan displays don't really have a "native" resolution, so this question does not really Apply there.

For LCD screens, as Duck said, typically the highest resolution the screen is capable of is the native resolution. You can get a list of supported resolutions using Screen.resolutions, however, this is not always guaranteed to actually contain the highest available resolution, if the user has set his desktop resolution to something less then the highest possible.

This has mostly historic reasons - in the days of VGA monitors, often the graphics cards would support higher resolutions then the screens, and sometimes the communication between the two, to find out which combinations are supported by both did not quite work correctly. For this reason, we would not actually list resolutions higher then the current selected desktop resolution, to avoid switching a system into a screen mode it does not support. I am not sure if this is the case on all platforms, though, and in general, the highest Resolution reported by Screens.resolutions is the closest you will get to getting a screen's "native" resolution.

If you want the resolution the screen is currently set to, you can get that with:

Screen.currentResolution

ie.

Screen.SetResolution (Screen.currentResolution.width, Screen.currentResolution.height, true);

Will set the view to fullscreen at without changing the resolution of you screen.

You can get the list of supported resolutions using Screen.resolutions.

I have a feeling that the highest supported resolution is very likely to be the native resolution if the screen is an LCD, however I'm not 100% sure of that.

I'm also not sure what that list will look like in the case of a multi-monitor set up.