Unfortunately there is no compass on our n900. It would make a perfect accessory for sky map apps and augmented reality apps, like in several modern android devices.
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.
