My alternator died recently and looking back, it was weak and giving me signs for a while. If you have a multi tester, set it to 20 volts and check the battery voltage with the car off. Red to positive and black to negative. Start the car and check again at idle. If it shows the same voltage or less with the car running the alt is DEAD. If the voltage goes up some but not much at higher engine rpms, it's weak and dying and only charging the battery at higher rpms and not at idle.

Naturally, the easiest way is to have a auto parts shop test it for free. I just feel funny trusting someone that wants to sell me what's being tested.

Les