Retina/High DPI support

[kegon]

I understand there was going to be an article about High DPI best practices but I have yet to find it.

I am going to try to follow the advice in
https://bugreports.qt-project.org/browse/QTBUG-23870
and
http://comments.gmane.org/gmane.comp.lib.qt.devel/6983

I have started to redraw my assets at 2x.

How can I support High DPI for images used in style sheets ?

And how can I know Qt is operating in High DPI, qApp->dpiScaleFactor() is not listed in the documentation for QCoreApplication.