The only true method of determining the charge state is to use a hydrometer. If it reads 1.275 specific gravity the battery is fully charged. It's that easy.
As written that is not even close to the reality. Contrary to popular misconceptions many people use hydrometers incorrectly too, no make that most people.
The electrolyte still needs to settle out over time just like any OCV reading does. 24 hours rest, no loads, no charging is what should be the ideal minimum for an accurate
SG reading. You then need to drain and fill three times per cell to get a clean cell reading and correct for temperature. Testing a battery shortly after charging will yield false SG readings. SG reading also tell you nothing about the batteries actual capacity at a given point in its life-cycle.
SG is best used to determine cell imbalances.. It is generally far safer to conduct an OCV test rather than risk acid spillage, burned clothes or a spill than to conduct regular SG readings. Once or bi-yearly is often a good protocol for wets to determine internal cell balance and to determine when to equalize the batteries. In a healthy battery all cells will have a nearly identical SG reading. Also "cheap" hydrometers can be VERY, VERY misleading. I personally use a far more accurate sight refractometer and when compared to a cheap
EZ Red hydrometer it shows just how inaccurate they really are. I have seen people throw away perfectly good batteries because of erroneous errors from cheap hydrometers. The floating ball version is a complete and utter joke, the EZ Red style not much better.
The 12.9v reading after 24 hours may not have been enough "rest" time or your meters resolution is bad or it's just a "new battery".. Some battery types can take well over 24 hours to fully settle out, especially when new.
With many wet cell deep cycle batteries 100% state of charge corresponds to 12.7-12.74V not necessarily 12.6. 12.6V is only 90% SOC for many wet cells.. The batteries that DavidPM bought are Deka Deep Cycle batteries, same as Sam's Club, West Marine, NAPA or O'Reilly Auto sells. Deka says any voltage above 12.6 can be considered full. Kind of non-committal..
I have however tested MANY Deka/East Penn batteries and found a resting voltage for "full" to be 12.7-12.73V.... With a new battery 12.9V after 24 hours is not out of the question. Apply a decent load for 5 minutes and then test and you'll be closer to reality or, just let it sit for a few more days..
These are Trojan's OCV numbers and I find they match up darn close to the DEka/East Penn deep cycle batteries: Note the 10% capacity difference between 12.6V and 12.7V!! On a 400 Ah bank that is 40 Ah's or a full day or more of capacity for many boaters....
Battery SOC at Resting Voltage and 80f (Source Trojan Battery)
100% = 12.73
90% = 12.62
80% = 12.50
70% = 12.37
60% = 12.24
50% = 12.10
40% = 11.96
30% = 11.81
20% = 11.66
10% = 11.51