Here are three ways to determine which way is true south.
This method uses the fact that the sun is always due south at solar noon. A shadow cast by a vertical object at solar noon runs true north-south. So, at solar noon, use the shadow cast by a plumb-bob string or the vertical edge of a building to determine true south.
To determine the local time that corresponds to solar noon, find the sunrise and sunset times from a current local paper (where "local" and "current" are both important!). Most GPS units will also give the times for sunset and sunrise -- just make sure the GPS is set to your time zone. Solar noon is exactly half way between the sunrise and sunset time. Note that the difference between local time noon and solar noon can be quite a bit, depending on your location in the time zone, and daylight saving time.
You can also use this NOAA solar time calculator to find the local time for solar noon at your location:
This Print Solar Noon Calendar prints out solar noon for your exact location for the entire year.
(thanks very much to Bill for suggesting this)
You can use a magnetic compass to find south. The compass reading must be corrected for the magnetic declination in your area. Here is how to determine magnetic declination in your area.
If you live in the southern hemisphere -- forget this one!
On a clear night, find the north star as follows.
Find the big dipper constellation.
Follow a line formed by the two stars that make the outer end of the dippers cup in the direction that the cup opens.
This line will intersect the north star at an elevation angle equal your local latitude.
North is in the direction of the north star, south is the other way.
You can use Google Earth to zero in on your house and find true south.
This requires downloading a small program from the Google Earth site. In the case of my house, the photo was up to date.
Place the item you want to determine the direction of right in the center of the display, as the angles chance as you pan around.
In the View menu, selecting "Grid" will display a latitude-longitude grid in which the grid lines run in the north-south and east-west directions.
In some areas, tax assessors offer downloadable maps of your property that show true south.
Thanks very much to John S. for suggesting these two methods.
Gary updated 8/11/07, 8/11/08, February 6, 2009