Use a compass, adjust for local declination (~-13.5° in Ballston Lake). Here’s how: Stand over a stake and find magnetic north. Turn around, keeping the compass over the stake, and look in exactly the opposite direction toward “magnetic” south. Now rotate your view, but not the compass about 13.5° to the west and pick a spot toward the horizon – this is Solar South. Put a second stake in line between your initial stake and the point on the horizon and now you have a true north/south line. I use this approach when evaluating sites, as timing is not important and even though natural variations in the magnetic field at this site may exist, it will be good enough for initial selection purposes.
Sunrise, Sunset and Solar South. Find “Solar Noon” by looking in a local paper for sunrise and sunset. Calculate the time exactly midway between the two. Be on your site a few minutes before Solar Noon. Put a fairly tall (about 6’ is good) stake in the ground and plumb it (level is handy for this). At exactly Solar Noon, put a second stake at the end of the first stake’s shadow. The closer you are to June 21, the shorter the shadow will be, so plan accordingly. This approach has the advantage of not needing a compass or knowing what the magnetic declination really is at your site. An accurate time piece (cell phone) is easier to come by. But, you need to be there at exactly Solar Noon and there must be enough sun for a good clear shadow! I used this approach for setting my house.
Note: +/-10° has a negligible affect on your solar gains (about -2.5%), but if you can be true Solar South, why not? When actually setting your house, there are more issues than just the sun, so keep in mind that even a +/-22° will only cause about a 8% loss of solar gain.
Notice in the picture below taken at solar noon that the shadow along the dormer is parallel to the wall. This means the house is facing solar south.