Finding the point at which the sun strikes your residence most often and most directly is key to passive solar and therefore to letting the sun do its work. For those who live in the northern hemisphere, this means finding true south. You probably have a general idea already, and frankly, your general idea is likely to be pretty accurate. Think where the winter sun rises in the morning and where the winter sun sets in the afternoon. Midway between those two points is true south. You could use the summer sun too, but it’s easier to gauge in the winter, because the points are closer together.
If you want to be precise, and you probably do, it’s easy. We once provided a great deal of information here about magnetic north, declination, and a host of other issues you no longer need to think about. Most any smart phone now comes with a good compass that’s based on GPS coordinates rather than magnetic north and therefore is as accurate as most of us would need.