I have three batteries that I overwinter each year (actually, two are 12 V car jars for backup purpose).
My hands on approach is this: Measure voltage at rest to see if they are within a range close to full charge (somewhere between 12.3 v and 12.6 V. I use a 12 V wall wart with a set of alligator clips. The wall warts non-loaded output is around 16 V. I attach the clips and monitor voltage on a daily basis. It usually takes three->four days to bring the measured voltage up to 14 V. At that point I remove power and let the batteries rest for a day or two. Voltage measurements at that time are around 12.7 V, fully charged. Repeat process every month or so.
