A common symptom of a bad alternator is your headlights will dim or even flicker when driving at night.
This means that your alternator is not putting out enough voltage or the voltage is starting to fluctuate and your alternator is likely on it's way to failure and needs replaced soon.
You car might also stall or become hard to start as well, and your battery might die often, strange noises might also come from your alternator.
There should also be an indicator light on your cars dash indicating whether the alternator is charging or not.
A way to test for a bad alternator is to start your car up and lift the hood and then remove the negative battery cable from the battery while the engine is running.
If the cars engine dies or starts to stall when removing the negative battery cable then your alternator is bad.