Some are sealed, but for the ones with removable caps I always remove the cap and check the acid level before putting them on the charger.
I have just one battery which always has low fluid in every cell when I bring it back for recharge. It takes an exceptional amount of demin water each time, at least 50 ml per cell if not more. All the cells are similar. But, when recharged, it reaches a reasonable voltage and certainly holds a useful amount of charge.
The cells can't be leaking significantly, firstly there is no sign of it and secondly this must have done a dozen cycles like this over the past year or two. I havn't actually checked the acid concentration, I probably should, and top it up if necessary.
Does anyone have an explanation?