You could do what I did… a load test. I have 2 6v GC2 210ah batteries in series.. so 210ah.. or approximately 100ah usable.
I put a 7a load (typical load for me) on and measured the voltage every 15 minutes. I also have a shunt that told me how many ah were used. I let the load run for 6 hrs and my voltage was at 11.7 under load. I stopped load and let battery recover. It came back to 12.1v. So that told me I was only getting about 50ah from my batteries… or half of what I should be seeing. I checked and one of my batteries was .3 volts lower than the other. Batteries were getting old, so I replaced.
This is not a perfect test, but it gives me a good idea of what battery SOC is under load, based on my typical load amps.