This is the closest I could come up with, using GetDeviceCaps() to determine the aspect ratio of the device corresponding to the desktop DC. A snippet from my code...
HDC hDC = GetDC(NULL);
if (hDC != NULL)
{
float dw = (float)GetDeviceCaps(hDC, HORZSIZE);
float dh = (float)GetDeviceCaps(hDC, VERTSIZE);
ReleaseDC(NULL, hDC);
// Equivalent of reducing a fraction
if (dw > dh)
{
dw /= dh;
dh = 1.0f;
}
else
{
dh /= dw;
dw = 1.0f;
}
wcp.fAspectNumerator = dw;
wcp.fAspectDenominator = dh;
}