A battery may or may not give out many discernible signs before it goes bad. There are symptoms that you can look for that would be related to identifying a faulty battery. The way to diagnose the problem decisively after spotting any battery-related symptoms is to allow a qualified mechanic to check the battery.
When you slide your key in the ignition and twist, the engine will not crank at all. You may only hear a clicking noise, which slowly fades away to nothing. These symptoms could signal a battery that is dead or one that has bad cable or terminal connections. The battery needs to be checked by a mechanic in this situation to determine if it is indeed the culprit. These symptoms can also be caused by the starter, or the alternator or generator.
A symptom that the battery is faulty is to turn on the car lights. If the lights do not illuminate when you try turning them on, it is a dead battery. Also, watching the lights while you or someone else starts the car can allow you to spot symptoms of a faulty battery. If the lights are dim or go dim during this time, the battery has a low voltage. Other related symptoms of a dead battery are when the car is off and you turn the key forward in the ignition. With the key in this position, the car radio will not come on, and the windshield wipers will not work.
A symptom of a faulty battery is when your car starts, but it does so slowly. This symptom is called a slow crank, when you need to hold the key twisted in the ignition for an extended period of time while the engine slowly turns over and starts. The battery is not yet dead, but it is low on juice and will eventually not start the car.