I just take a 4-5' piece of smooth wire and shove it in the ground about a foot. Pick a place directly under the fence.
Now get stick that you can use to touch your grounded wire to the hot wire. When they meet you will see a little spark and hear a little zap.
Leave the wire in the ground and check it whenever you want. Obviously make sure that your ground wire only contacts the hot when you want to test.
