The battery voltage itself is somewhat irrelevant, what's important is how much charge is stored in the battery and this can't really be tested fully. The best way to test the serviceability of a battery is using what's called a 'drop tester' which puts the battery under load whilst checking the voltage to see if the 12Vs can be sustained.
Even this is reasonably pointless as just by starting the engine you're going to get equivalent results. If your car starts okay each time, even after leaving it for a few days, then it’s serviceable.