There are a number of ways geologists can measure the state of stress in the earth. One way is to drill a hole and then to put strain meters in at various depths. As the rocks deform the very tiny movements of the rock can be measured. This is called the strain. If these are continuously monitored then the strain rate can be determined. This is called borehole geodetics.

Another method is to measure deformation using satellites. This is done by radar beam from the satellite to the surface. The radar beam reflects off the Earth back up to the satellite. The travel time can then be used to measure distances very accurately. By making multiple passes the deformation at the SURFACE can be measured.

It's difficult to measure the stress directly - pretty much all we can measure is the effect that the stress creates, in this case, quakes. We know how fast North America is moving relative to the Pacific plate, and this tells us what the rate of stress buildup is. If you divide the rate of stress buildup by the size of the earthquakes that dissipate that stress, then you can calculate how frequent these earthquakes will be - but, apart from observing that certain types of faults generate earthquakes of a certain range of sizes, we can't really predict how big these earthquakes will be.