If you mean via DDC then I guess this should work in most places, but for whatever reason it does not work here :( In particular, xdpyinfo says:
screen #0: dimensions: 1024x768 pixels (321x241 millimeters) resolution: 81x81 dots per inch
So it appears to know the screen size (I measured with a ruler and 321x241 is right), but it gets the DPI too small anyway. At 81 dpi the text is hardly readable.
But: 1024/321*25.4 = 81.026 768/241*25.4 = 80.943
Seems to me X has it very right. That Windows usually doesn't care about real resolutions and instead fudges with it (consider the Large Font setting in the Display Control Panel) is probably another issue. Or the metrics this function requires are not dots per inch but something else.
Rolf Kalbermatter