The San Andreas Fault runs from lower right to upper left through this picture. Image by Fairchild Aerial Photography. Geology can also be used to find faults in the wild. By mapping the location of various rock units, and by noting their characteristic geologies, you may find very similar rock units which are hundreds of miles apart. And you may find rock units which are right next to each other and clearly could not have formed so near to one another.
Geophysics can help locate faults as well.
To find faults which only rarely have earthquakes or are no longer active, one can use gravimetry and geomagnetic measurements to find linear features in the gravitational and magnetic fields of the Earth. The figure below shows a nice image of the anomalies in the Earth's magnetic field caused by a series of faults near Portland, Oregon. Map of the intensity of the geomagnetic field near downtown Portland, Oregon. The colors represent the relative intensity of the magnetic field. The dashed lines were drawn to emphasize faults known from previous geologic mapping.