How do you determine the difference in time between the arrival of the primary (p) wave and the arrival of the secondary (s) wave during an earthquake?

Primary (P) wavesa type of seismic wave that involves alternating compression and expansion of the material through which it passes. 
6 to 13 kilometers per second, these waves arrive before S waves. 1.7 times faster than S waves. These waves are compressional.Secondary (S) wavesa seismic wave that involves oscillation perpendicular to the direction of propogation. 
second to arrive and bigger amplitude than P waves. These waves are shear.