Easiest way is to take them in to autozone, oreilly's someplace like that.... have them load test them and they can tell you if they're good or not.
For at-home testing, what I'd do is unhook the batteries when you're going to leave it set for a while....when you hook it back up if it cranks, you might have something that's draining the battery, you just have to track that down. If it doesn't start after being unhooked, you know your batteries are dead...be sure to replace both at the same time....