Switching parts of the distribution system off is the only way that I know of to find a ground fault.
Just last week we found a ground fault on a Delta system that we had been looking for since last October.
Unfortunately if you have multiple ground faults on the same phase you may never be able to track it down (it's like the old series connected Christmas tree lights where any single bulb going out kills the whole string).
One of the reasons that the Delta system is good is that the equipment will continue to operate if a ground fault occurs.
One of the reasons that the Delta system is not so good is that the system will fail in a particularly spectacular fashion if a 2nd ground fault occurs on another phase.
