This is a frequent question. Is it a battery going bad or an alternator not charging? There are devices that plug into the vehicle's 12VDC receptacle. With the engine off, the voltage read out is the 'battery. Ideally it should be 12.6 VDC or so fully charged. When you start the vehicle, the voltage read out is the alternator output. It will be 13.4 or higher. 14.4 VDC is not unusual. If the 'engine running' VDC read out is fluctuating wildly , it could indicate that the alt's voltage regulator is bad. Some small fluctuation is normal: it is the regulator maintaining a set voltage.
These devices are not expensive. They run from $10 to $20. Here's one from Amazon as an example, not a recommendation. These now often have a USB charger built in.