Amazon has hydrometers for less then $10.. One of these showed me that my $OMG tractor battery was getting sick even though it showed 11.9vdc at rest and charged to 13.9 when running. Only drawback is you have to have old school lead acid "open" cell batteries to take advantage of it 
I thought about bringing up hydrometers but I have found over time that people will not go to the trouble to check their batteries so I gave up recommending them. Heck, most people will not bother to check the electrolyte level or fix terminal corrosion. A hydrometer will give you advance warning that a battery is headed south. When single cell starts reading appreciably lower than the rest of them you may as well break out the piggy bank or else make sure your SeaTow membership is paid up.
11.9 volts at rest is a sign too. After charging batteries, let them sit for an hour then check them. If they are much less than 12.6V then they are not 100%.
Advance Auto sells hydrometers for under $5. They are not always accurate as far as the reading goes but they are repeatable. With the cheap hydrometers a new battery may show in the yellow instead of in the green. If you know where your batteries normally read with that hydrometer then you will still be able to tell when a cell is reading low.
