I don't recall the source, but they identified the CRS-7 strut by using several mechanical vibration sensors placed on various parts of the rocket. The deflect

I don't recall the source, but they identified the CRS-7 strut by using several mechanical vibration sensors placed on various parts of the rocket. The deflection / explosion reaches the sensors at different times, and with precision timing, it enables you to reverse pinpoint where it must have originated: the failing strut.