Buy a voltmeter. Check the terminal of the battery when engine is running, Voltage should read 13 volts or above. If it is below that then you have an alternator problem. If ok,Check your battery after shutting off the engine it should read in the range of 12 volt,then check it again after an hour it should not drop dramatically if it does check again the battery this time with the negative terminal removed.If it still dropped then battery is at fault and if not you have a ground problem in your electrical.

Take a battery terminal off while car is running if it stays running it is good if it does the. Alternater is bad.

You can have it check at an auto parts store . No charge.

There will a be a light on the instrument panel to tell you the alternator is bad. You may also notice flickering interior lights and headlights, a humming sound from under the hood, the smell of hot or burning wires, And the battery dying frequently.

Some vehicles also go into limp mode when the alternator completely fails.

Go to Auto-Zone, and they will put a "multimeter" on it while running; SHOULD read ABOUT 14.25 Volts !

the battery light comes on

A red ALT light shows up on the dash.