It can be very hit and miss, I find, I have had the DNO chap pull out his laptop with a map on it when confronted with a fault, the map indicated an overhead service, when it was cleary underground, now it might have been overhead originally but likely to have been moved underground in the 80s...
I think if there is doubt, they are required to attend site to locate cables for folk who need to dig in the ground and I think it is FOC? I know of one site where a HV cable was exposed by ground workers, site maintenance rang the DNO who attended, found that it was suspected to be a dead section left in after a diversion, and they had it spiked later on that afternoon, it was indeed dead, but was quite impressed with the speed of response, considering that the cable had not been damaged by the groundworkers and digging had been suspended