You don't. OpenGL does not provide any interface for accessing this memory directly. You can get access to the contents of those buffers by using glReadPixels() but it is slow. There has been much debate over whether this is a good thing or a bad thing, but the reasons are basically irrelevant. You can't get direct access through OpenGL. Access to these buffers belongs in the windowing system (through DirectDraw or something similar) and not in OpenGL. Currently Windows does not provide any means to access this information.