There's several ways to test, and most only tell part of the story. I use a special eye dropper, that has tiny floating plastic balls. This thing is supposed to measure specific gravity, or something. What it does is tell you about the condition of the battery acid. The instructions are on the side, and it's very simple to read. It's exactly like the ones that measure anti freeze, by counting how many balls are floating. Don't know for sure, but I doubt if they're interchangeable. But they only cost about two bucks at walmart, so it doesn't really matter.
Also check voltage. AutoZone, and the like, will do this for free. But I use a cheap tester from harbor freight. It's one of those that use led lights, and costs about five bucks.
Put a good charge on that battery, and test it. The easiest way is to turn on the lights for a minute or so, to break the surface charge, then test. Or just test it after it's set up a day, after charging.
Buy a battery tender at walmart, then you'll have a battery maintenance charger. But it won't charge a very low battery. It's not really a charger, it's a maintainer.