Really the only true test of state of health, for cycling batteries, is to test them by doing a 20 hour capacity test.
How to conduct a 20 hour capacity test:
#1 Fully charge battery then allow it to rest, disconnected, for 24 hours
#2 Make certain battery temperature is between 75F & 80F
#3 Create a DC load = to Ah Capacity ÷ 20 (small light bulbs and resistors can work)
#4 Start DC load and a stop watch at the same time
#5 As the voltage drops during discharge adjust the DC load to maintain as close to C/20 rate as is humanly possible.
#6 Stop the discharge test when battery terminal voltage hits 10.499V
#7 Note the hours and minutes of run time and figure your percentage of 20 hours that it ran. This is your batteries Ah capacity or state of health as a percentage of as new rated capacity. For example if a 100Ah battery ran for 16 hours it is at 80% of its original rated capacity. Lead acid batteries are generally considered “end of life” when they can no longer deliver 80% or more of their rating but can sometimes continue to "work" down into the 70% range before failures start to occur..
#8 Recharge the battery immediately.. Follow this up with equalization level voltage and measure specific gravity until all cells match. (not for non-Lifeline AGM's or GEL's). A long slow recharge can have a slight reforming effect on flooded batteries and can actually recover some lost capacity.
Is 10.5V bad for my batteries?
A once yearly discharge test, done correctly, is actually less damaging than taking a battery to 50% SOC and leaving it in that range for a day or two, or the PSOC cycling (partial state of charge) the average boater thinks nothing of. Regular PSOC cycling is more damaging than a once or twice yearly Ah capacity test. A capacity test, done correctly, simply counts as another deep cycle.
WARNING: The only time your batteries should regularly be taken below 12.1V is during a capacity test. For regular house use, at your average house loads, your deepest loaded voltage should ideally not dip below 12.1V or better yet 12.15V - 12.2V. Unless you are running short duration high load device such as an inverter, windlass, electric winches, thruster, water maker etc. don't let your bank voltage dip below 12.1V....