For most cars it is very likely a faulty alternator, to be sure you need to use a voltmeter. Measure the battery voltage while the car is running. It should be at least 14 volts while the engine is running at 2000 rpms or more. A voltmeter is an indispensable tool used to diagnose automobiles, you can buy an adequate one for less than 20 dollars.
If you don't want to do that, you can remove the alternator and take to a auto parts store that tests alternators. It's usually free.
Remember I said most. There are a few cars that can throw you a left curve. If the car is old enough to have an external regulator, (mid eighties and older), or is a Honda, it is a bit deeper discussion.