How to test insulation of an unarmored, buried cable with mega-ohmmeter?
A fail on an insulation test tells you a cable is bad but a pass doesn't tell you it's good.

Banging in a ground rod and testing too it may help find some faults but if the cable is not yet burried or if the soil it's packed in is too dry or loose it may well leave issues undisocvered. 

I can't give specific advive on regulations as I don't know what country you are in but the whole situation seems dodgy as heck to me.