Most auto places
load test batteries for free. That's really the only way to know for sure. That said, you can do some stuff at home:
1) If there is a .050 (sometimes expressed as 50 "points") or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell(s). Using the battery manufacturer's recommended procedure, applying an Equalizing charge may correct this condition.
2) If the battery will not recharge to a 75% or more State-of-Charge level or if the built-in hydrometer still does not indicate "good" (usually green or blue, which indicates a 65% State-of-Charge or better).
3) If a digital voltmeter indicates 0 volts, there is an open cell.
4) If the digital voltmeter indicates 10.45 to 10.65 volts, there probably is a shorted cell. A shorted cell is caused by plates touching, sediment ("mud") build-up or "treeing" between the plates.
5) Load Test the battery as recommended ...
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