Get some analogous clock app and point the hour hand at the sun. South (or north - depending on the hemisphere) is between the hour hand and 12 o'clock.
You have to correct that for the deviation of your local longitude from the one of the meridian that defines your time zone (4 minutes for every degree) which is easy because you have a GPS receiver.
The Orrery app will show you the azimuth of the Sun, Moon and planets, on a compass-like display. So if you have a clear sky, you can use it as a compass.
