Microsoft made it very simple to get information about the displays available to Windows with the Screen class in Windows Forms. Using this class you can identify the primary screen, and get properties like size from any display device.
For anyone developing an application which supports multiple monitors the Screen.Bounds property may be useful in finding screen offsets. If you have multiple monitors attached try the Get Current Screen Info button on your secondary display.
Download the C# sample code and binaries to explore further. Like all good API examples I have omitted error handling ;)