As a general rule, I remove my boat's two batteries during the off-season and store them on a piece of plywood in a dry, warm place in my shop. I check the batteries and charge them each month. But before I put them back into the boat, I take an additional step: I transport them to a local store that sells batteries and ask for a load test.
My batteries are 12-volt marine units rated at 850 cold-cranking amps. The store's workers run the battery to 425 amps (half the CCA value) and monitor the voltage for 15 seconds. The voltage should not drop below 10.5. If it does, it's time for new batteries.
Most shops don't charge for this test, and it could save you valuable time and aggravation later should your batteries fail to start on that first spring fishing trip!