The 12V battery in my John Deere L120 has two covers on the top that can be popped off, so I'm assuming distilled water can be added if necessary. My question is, how do you determine if water needs to be added? In other words, what is the correct water level?

If you see a vent slot in the hole, make sure you leave about a quarter inch space at the top of the slot, or see if there is a fill level mark on the battery.

When you look into a battery that needs water the water will be perfectly flat and a very dark color because it is sitting down low in the battery. As you add water it will rise and when it reaches the correct height it will become dish shaped due to surface tension causing the water to rise higher around the edge, it will also look brighter because it can reflect more light back to your eye. In other words, what Mowerman said.

Both of you were right on the money. The distilled water dished and the level is 1/4 - 1/2 inch from the top of the vent slots. It's on my charger right now; JD Dealership told me the battery is still good, so I'm hoping this does the trick. Thanks.