You may want to consider the BENDER EDS3365 Portable Ground Fault Locator for ungrounded AC and DC systems. Many utilities are using it to detect ground faults in their 125/250V DC systems. Information is available at www.groundfault.com under the products section.
I would recommend using a Bender Ground Fault Monitor, e.g. IRDH275 with a voltage coupler, e.g. AGH520S. It measures the resistance level of the ungrounded system and alarms when the resistance to ground drops below the alarm setpoint. Also may want to look at RCM470DY Directional Ground...