http://blogs.msdn.com/jaimer/archive/ 2007/03/07 / Get-system-dpi-in-wpf-app.aspx похоже работает
PresentationSource source = PresentationSource.FromVisual(this);
double dpiX, dpiY;
if (source != null) {
dpiX = 96.0 * source.CompositionTarget.TransformToDevice.M11;
dpiY = 96.0 * source.CompositionTarget.TransformToDevice.M22;
}