MonitorFromPoint
Determines the monitor that contains a specific point.
MonitorFromRect
Determines the monitor that contains the largest piece of the rectangle.
MonitorFromWindow
Determines the monitor that a window belongs to (which monitor contains the majority of the window).
GetMonitorInfo
Gets metric information for a particular monitor.
ChangeDisplaySettingsEx
Same as ChangeDisplaySettings except that it allows the caller to specify the device.
EnumDisplayMonitors
Returns info about multiple video devices that an application may overlap. A monitor need not be part of the Windows virtual desktop.
EnumDisplayDevices
Allows you to determine the actual list of devices available on a given machine. You can enumerate the devices and get a DC for any monitor on the system that Windows supports. This allows an application to work with a dedicated display device without sharing the screen with other applications. (This is used, for example, by the Control Panel.)
SystemParametersInfo and GetSystemMetrics
These APIs were updated slightly to include support for multiple monitors, and CreateDC was updated to take a display name in the \\.\DisplayX, where X is a number. This can be used in conjunction with EnumDisplayDevices to get DCs for specific monitors.