alt Hacker NewsLocal people know where they are because once upon a time a D8 drove through pulling a big orange spool of fiber conduit and passers by chatted about how another fiber was going in. Also around here the contractors seeded the disturbed ground with a wildflower mix (presumably to discourage weeds) so you can also look for straps of wildflowers.
And of course there are marker posts that say "buried fiber optic cable do not dig".
Once you get out farther from civilization these signs become less common and there are more situations where they can get removed.
Most cheap hired labor doesn't realize you shouldn't dig up the market posts and throw them in the dumpster on construction sites. Add a little bit of rain and suddenly the contractor showing up with an excavator that was told everything was properly marked, and can see markings in other places but not where they are digging, and disaster occurs.
Also soils aren't static, I've been on sites where the midpoint of a buried telecommunications cable had drifted over 8 feet from where the markers were a few hundred yards apart. Just looking at the markers and assuming a straight line isn't a safe bet. Looking at the fence rows to the left and right of the cable and you could see a matching bow in the fence rows.