Check the battery warning light on the dashboard
Inspect the alternator belt for looseness, cracks, or wear
Start the engine and listen for unusual noises near the alternator
Use a multimeter to measure battery voltage with the engine off
Use a multimeter to measure battery voltage with the engine running
Check for voltage around 13.5 to 14.8 volts with the engine running
Turn on headlights, blower, and other electrical loads and recheck voltage
Look for dimming lights or weak electrical performance
Inspect alternator wiring and connections for corrosion or damage
Test the alternator output at an auto parts store or repair shop