Re: AGM batteries
You can load test them. Fully cycle them, charge them 100% and let them stand, then put a load test on them. You can buy load testers under $50 for something that small on Amazon (3x higher at the local auto store) or maybe borrow one for free from Autozone or Pep Boys tool rental program. Or take the batteries into a shop and ask them to do a load test, usually free.
You can also load test them by fully charging them, then connecting a load like a headlight and timing it until the voltage drops. Or fully charging the batteries, letting them stand overnight, and then using a good accurate digital multimeter and assuming 12.6 was a new battery, derate them 10% for each 1/10th of a volt they are below that. That's not perfect, because meters aren't dead accurate and 12.6 isn't a universal truth, but it will give you a "1/8-1/4-1/2-3/4" type profile versus the label rating.