[OAC_Sparky]

This is a general guide to non-technical troubleshooting; based on symptom:

Generally speaking:
-If the car is running fine with all the accessories and stereo and lights running, the alternator is likely fine.
-If after running the car for a while and the battery starts the car OK, the alternator/voltage regulator is likely OK.
-If you run the car for a while with the headlights on and all is OK, and you shut the car off and the headlights dim appreciably (a lot) it points to a charging problem (battery/votage regulator/battery connection)
-If you run the car for a while with the headlights on and all is OK, and you shut the car off and the headlights dim only a bit (not a lot) the charging system is probably normal; BUT when you go leave the car overnight and the battery is low when you go to start the car you either have a battery problem or a voltage leak somewhere (see my post above).

Also: look at your dome light when you crank the car ie turn it on. If it fades (goes dim) as you're cranking it's a low battery. If it doesn't dim a lot but the starter cranks slow you may be looking at a starter/starter wiring/starter solenoid problem.