Your alternator keeps all the electrical processes in your car running. It does this by converting mechanical energy from the crankshaft into electrical energy stored in the car's battery. The radio, lights, ignition systems, battery and other electrical processes may fluctuate in reliability before the alternator actually fails. It's important to know the appropriate signs so that you can diagnose a failing starter (or alternator) before it actually goes out at an inconvenient time.
Check the battery's voltage output with a DC voltmeter. If it measures below 13 volts consistently, or appears to be diminishing in power outage, it could mean a bad alternator. The other possibility is that the battery could be failing. Take the battery to a mechanic or the car dealership where it was purchased to have it tested.
Take note of the reliability of your electrical accessories. If the radio, interior lights or any other electrical accessories appear to be working inconsistently, it may be a sign of diminished battery electrical output, which, in turn, is a result of a failing alternator. If ignored, this will gradually lead to a shutdown of all electrical processes.
Check the dashboard indicator or service light. It's located in a slightly different place in every car, but it is on the dashboard along with all the other warning lights. Check your car's user manual to locate this light. If it's not supposed to be on and it's lit, don't ignore it. A flickering light signifies fluctuating amounts of battery currency and that the battery, alternator or LED light needs immediate attention from a mechanic.
Listen for odd sounds. A clanging or rattling noise may indicate a loose alternator belt or some other broken part. A clicking noise when you put your key in the ignition and try to start the car indicates that your alternator is dead and needs to be replaced. The warning sound that you'll hear before the clicking is a "whir, whir" noise, followed by a sluggish starting engine. Listen carefully to all noises, especially if additional signs of a bad alternator are present.