Batteries Plus, now known as Batteries + Bulbs, will perform a free "capacity test" not to be confused with a "cold crank amp test" that would be done for automotive batteries.
They did this for me on six deep cycle batteries, one was at 9%, the best one was at 86% (if memory serves), 86% they consider still good. They recycled the bad batteries for me, also at no charge.
They are, of course, hoping you'll buy from them, which I was so grateful for the service I purchased two replacements, but you don't have to.
This test is time consuming, be prepared to leave your battery with them for a day or two, if no one is ahead of you in line for the test. I took two in at a time until I was done so I had the others to use while the others where being tested.