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.
Sam,
I will try to remember that one.
I'm a long time blade of grass user though. And cheap. I might find a different use for the wire.............grass is free!
Geno