GPS satellites can very precisely measure markers on earth's surface. These markers can be set up on opposite sides of the fault to detect BOTH vertical and horizontal movement at faults. So, GPS satellites can detect movement at reserve, normal, and strike skip faults.
Friction between rocks on either side of the fault causes them to stick together. This causes stress to build up. Eventually this stress causes the rock to move and break.
No, geologists cannot predict exactly when earthquake will occur. We can monitor movement at faults, but that does not mean that we can determine precisely when and where the earthquake will occur.
A seismograph is a device that measures seismic waves. For the seismograph to work a drum with paper rolls to release more paper, and a pen stays in place to record any movements in the ground on the paper. If the ground moves, the machine and paper move so that the pen makes lines up and down on the paper showing the time and amount of ground movement.