I don't know if it's relevant for these AGM types of batteries, but you can take lead acid batteries to many auto supply stores and they have a gadget to test them under realistic load. Even walmart can do this.
Once, I thought my battery was going out and I happened to be within walking distance of a walmart. I was going to just buy a new battery on the spot, but they tested the old battery and said it was good. They printed a little slip of paper with some specs and tests the battery was capable of passing. I would bet autozone or similar could do the same. Worth calling one of them to find out.