Here in Texas we prize the water-addable batteries - they last 7-8 years vs the non-serviceable kind, which dry up in 2-3 years.
There is usually a formed-in split ring about an inch down in the cell which is the water level indicator. But, if the plates are well covered, then water level is probably not your problem.
The most common falilure mode for a battery with sufficient water/electrolyte is to still indicate 12.5 volts or more at rest, but it cannot take a load, and will drop to 5V or so when trying to start; and even the power windows will run slower. You can put a voltmeter on it, hit the starter, and you will know - if it drops below about 10 volts when cranking, it's a goner. We've had two with these exact symptoms this week.

DG