I've not done any digging to check, but I would presume along the routes where they install the "buried fiber optic cable route" signs they also install the marker tape.
That or the blinding blue flash from the hole...
I would expect the marker tape is pretty much non existent for most residential drops.
I expect in many cases people don't even notice they chopped a wire and don't figure out until they try to make a call or notice the phone hasn't rung in a loooong time...