First, get a voltmeter. Next, start using it.
Then, get a wattmeter for the bike, charge thru it, and record the stated results. You could get a wattmeter for the wall outlet on the charger but then you would need to know the charging profile and electrical efficiency of the charger, which is not AT ALL a simple thing to measure. Notice how the charger gets warm? That is wattage being USED, by the charger, but NOT going into the battery.
After that, spend a few weeks analyzing watts IN, and watts OUT, and comparing those numbers with voltmeter results, voltmeter being used immediately after a ride and again an hour or two afterwards. Also immediately BEFORE a ride.
After doing all that, and ONLY after doing ALL of THAT, will you have some basic understanding of how your battery is currently performing.
Then, you will need to learn that the performance observed, declines steadily and fairly consistently, every day, until one day the decline will start to accelerate, and some time after that, you need a new battery.
While you are at that, you should also learn that there are several behaviors and procedures followed by you, the end user, that can and will accelerate or slow down that decline.