The Gallagher B1200 electric fence charger has a high (H), medium (M), and low switch on it (L). If you have it set on high the fencer will read around 6Kv to 7Kv or a little higher. You need to make sure that you're using an electric fence tester that's designed to test for electric fence voltages and not using a standard AC DC volt meter, you'll either burn up the meter or get a fluctuating of numbers, which won't make any since. You'll take the ground probe of the tester and put it on the Green terminal of the B1200 and the other part of the tester on the Red terminal of the B1200, then turn it on to the high switch setting and see what voltages you're getting on the screen of your tester. If it's upwards to 6Kv or higher your fencer is good, if it's quite a bit lower then there's something not quite right with the B1200.
The fence voltage can vary in a good way or bad way.You'll use the same tester you used one the B1200 to test the fence. You'll take the ground probe of the tester and push it into the ground and lay the hot part of the tester on the fence itself. If it shows 6Kv or higher then you're fence is in good shape. If it's between 4Kv and 5Kv, you're fence is in OK condition, but you may have some shorts dogging down on the fence or your ground system is inadequate for that size of charger. If the voltage is below 4Kv, then you definietly have some issues going on with the fence.
The B1200 is quite a powerful fencer, definietly don't want to grab the wire espescially if the B1200 is working good. There's a trick of trade from the old days for testing the electric fence without using a tester and without grabbing the wire with your bare hand. What you'll need to get is a longer piece of green grass about 8 or so inches long. Squat down to a knee, pinch one end of the blade of grass between your thumb and index finger and lay the other end on the fence wire. You should feel a slight ping in the blade of grass. As you move the blade of grass inwards the ping will get stronger, so be wary of moving the blade of grass to close if you can already feel it. The green blade of grass will eat up about 90% of the shock.
If I was to be looking into a good fence tester that will do fine for reading voltages I would go with the Gallagher DVM3. You can look at your local farm store that carries Gallagher or go to the following link.
Hopefully this helps you out and points you into the right direction.