Ideally you will have the battery tested with an electronic tester, but you can play detective, how old is the battery? (If over 6 years old I would replace) how can I tell how old it is? If it says look in the owner’s manual on the top it is the original factory fitted battery, some batteries have a recharge date on them and normally the battery would have been sold before this date.
If it has a built-in hydrometer, and it tells you it is fully charged (shows green) but it won't start the car we know one of the 6 cells has failed, as it only monitors one cell not all 6 (a good battery can't be flat and fully charged). If you have a volt meter connect to battery and observe the voltage when trying to start the car, if it reads 12 volts with no load but drops to a very low voltage when trying to start the battery is faulty. A good car battery that needed charging would read maybe 11.5 volts.