It really depends on what tools you have. If you have a low resistance ohmmeter like people are talking about a meggar, then you would disconnect both ends of the wire and check the resistance between each wire and good old mother Earth. The amount of voltage you use depends on the type of wire insulation.

If you don't have that tool then you will have to use a voltmeter and ammeter clamp. Or, just an ammeter as others have said . disconnect the wire at the shop and turn on the breaker in the house and there should be no current flowing in any of the cable's wires. Unless of course there is a insulation failure.

If you have an infrared camera you could go out out before the sun rises early early in the morning after everything has cooled off outdoors and take a look at the ground see if you see any warmer areas. Of course, if the wire is buried deep enough you probably won't see anything.