Time Domain Reflectometry is used to find faults in cable lines. A pulse is sent down a length of cable, where the impendance is known. When an impedance mismatch is encountered, part of the pulse is reflected back. Using mathematic calculations, the distance to the impedance mismatch (fault) can be calculated. The velocity of propogation of the dielectric in this cable must be known to calculate the speed that the pulse travels at.
For fibre optic cable you would use an OTDR (Optical TDR).