You don't test a battery with a digital multimeter, you test the electrolyte with a hydrometer as posted above and you also test with a "load bank" that most parts houses have to test batteries.
A load bank basically makes the battery power a large heating element and a fan to simulate a 100amp current draw while it simultaneously measures for voltage drop. If it drops below spec during the load test it's garbage.
Using a digital multimeter to measure voltage without any kind of load on the system doesn't tell you much really.
Testing the individual battery cells with a battery hydrometer can tell you if one or more cells are weak and are not accepting a full charge.
During three repeated tests of my five year old battery over the course of a month last October produced these results on one battery cell.
I had the dealer replace my battery during a routine oil change and when it was two below zero here in NJ last Saturday my vehicle started right up.