May I recommend buying yourself one of the many inexpensive battery testers that are on the market. There are several of them on the market that can be had in the $50 - $80 price range.
I went with the Solar BA9. It has served me very well. I like it. Foxwell offers a couple inexpensive models, including the NT301, which is also a simple OBD2 scanner. So does Ancel. I recently tried an Ancel BA301. Nice features and very easy to use. I returned mine because it wasn't accurate. But I've seen reviews where others have tested the BA301 against several other devices and found it to be accurate. I may have just got a bad one.
By using a battery tester as part of your regular car maintenance, you can track your battery health, and have an idea well in advance, before a battery totally fails. My battery tester recently saved me from some battery grief. During a cold snap in late December, I decided to charge and test the batteries in my cars. The H8 battery in my E350 tested bad, even after charging. It was still starting and running the car, but I got a replacement. Fortunately, it was still in warranty, so Pep Boys put a new battery in my car. Without the tester, I could easily have been stranded, in miserably cold weather.