How to detect the current screen resolution?

• Size of the primary monitor: GetSystemMetrics SM_CXSCREEN / SM_CYSCREEN (GetDeviceCaps can also be used)
• Size of all monitors (combined): GetSystemMetrics SM_CX/YVIRTUALSCREEN
• Size of work area (screen excluding taskbar and other docked bars) on primary monitor: SystemParametersInfo SPI_GETWORKAREA
• Size of a specific monitor (work area and "screen"): GetMonitorInfo

Edit: It is important to remember that a monitor does not always "begin" at 0x0 so just knowing the size is not enough to position your window. You can use MonitorFromWindow to find the monitor your window is on and then call GetMonitorInfo

If you want to go the low-level route or change the resolution you need to use EnumDisplayDevices, EnumDisplaySettings and ChangeDisplaySettings (This is the only way to get the refresh rate AFAIK, but GetDeviceCaps will tell you the color depth)

When system use DPI virtualization (Vista and above) using GetSystemMetrics or GetWindowRect will fail to get the real screen resolution (you will get the virtual resolution) unless you created DPI Aware Application.

So the best option here (simple and backward compatible) is to use EnumDisplaySettings with ENUM_CURRENT_SETTINGS.

It's GetSystemMetrics with these parameters:
SM_CXSCREEN < width
SM_CYSCREEN < height

As it says (SM_CXSCREEN):

The width of the screen of the primary display monitor, in pixels. This is the same value obtained by calling GetDeviceCaps as follows: GetDeviceCaps( hdcPrimaryMonitor, HORZRES).