you can rent a time domain reflectometer for this purpose… it sends out a signal and then listens for the echos back and calculates the distance. problem is they arent accurate the first like 500 feet so you have to add a reel of 500 ft of matching cable to the end first. electrorent.com rents them.
this is a great untapped business idea. people need an idiot proof but safe and yet a succession/trust plan. i struggle with phone backups too.