You will need to conduct a car battery test using either a special battery meter (or a multimeter).  A fully 100% charged battery should register 12.66V or higher on your voltmeter.  If the voltmeter reading is below 12V, your battery is no longer good and it would be wise to replace your car battery.  If the reading is <11V, your car battery is about to die and you should change your car battery right away.