A method and a related apparatus for monitoring an external surface of stranded cables, such as submarine cables for optical telecommunications, and electrical energy cables, provide for telemetrically acquiring a profile of the cable external surface by means of a laser optical triangulation technique, obtaining a spectrum of the telemetrically-acquired profile and comparing the obtained spectrum to a previously measured reference spectrum corresponding to a cable free of defects, thereby deducing the presence of defects by differences in the spectra.