Eventually I just created a small cocoa function to return this value for me. I use it to determine the time of paintEvent whether I should use hiDPI images. Works like charm on my MacBook Pro 15" Retina.
bool MYAppCocoaServices::isHiDPI(QWidget * widget)
{
NSView* view = reinterpret_cast<NSView*>(widget->winId());
CGFloat scaleFactor = 1.0;
if ([[view window] respondsToSelector: @selector(backingScaleFactor)])
scaleFactor = [[view window] backingScaleFactor];
return (scaleFactor > 1.0);
}
I am building this .mm file conditionally on Mac only and call this static function from my c++ code on Mac.