First things first, we always recommend charging your phone to see if it can reach full battery. If it is unable to reach 100%, chances are the battery needs to be replaced. Next, test to see how long a fully charged phone can remain operational. If you have limited phone and app use and the battery still drains within a few hours, chances are it needs to be replaced.
Lastly, check the settings in your phone. Most phones now have a battery health indicator to show you how much longer it has before needing to be serviced.