Let's try going at it another way.
1. Whenever you measure 13.8 volts or more on the terminals of a flooded lead acid "12V" battery, it is under charge or, perhaps, still has a bit of float charge left over from recently removed charging
2. Whenever you measure less than this amount, the battery is either resting without load or is under load.
I'm curious as to why you're asking the question. Modern smart chargers adapt their charge to the battery condition they sense.
IMHO, typically, charging
voltage is TOO LOW. To get a properly and fully charged flooded battery, you really need upwards of 14.2-14.4 at room temperature or, better, with pulse charging you can routinely sustain 15V or more.
TIME is perhaps the most important component in achieving a full charge. The final 10% or so takes a long time, and is important because if the battery is left in less than a fully charged condition for very long, it begins to sulfate. Sulfation (the formation of PbSO4 crystals on the plates) will in time reduce the capacity of the battery to store and to deliver it's designed energy level. As Rick suggested, you won't be able to monitor capacity
with a volt meter. You can have a seemingly fully charged battery showing 12.6-12.7 VDC at rest which has a badly depleted capacity --
due to sulfation and/or other factors --
and which will very quickly go dead when a significant load is applied.