You should add 1 inch of water to the battery at the top. You should also check the battery at least once a month. If you keep checking, you will notice when the battery needs water.
If the battery is exposed to water, you can see the water dripping. If the battery is not exposed to water, it is best to check the battery once in a while to ensure it is not exposed to water. Just like cars, golf carts can also be exposed to water. You should check for water and ensure that it is not flooding the battery.