When your battery does not get charge.
Typically you know your alternator is out when you have to jump start your car numerous times. A way you can test it is by turning on your car and while it's running discount the + terminal. If the car turns off then your alternator is bad. If the car remains in then alternator is good and possible bad battery. Hope this helps
If it is makeing a noise or is not putting out at least 13.5 volts it is bad. Take the car or the alternator to an Auto Parts store and have them load test the alternator to be sure.
with vehicle running pull posative side battery terminal off if it quits alternator is bad
A bad belt, a bad regulator in the alternator or bad brushes in the alternator.
To test the alternator if funds are low, let the motor idle and remove the negative battery cable, if the motor dies its probably the alternator.
Check voltage with a voltmeter at the battery With engine running readings should be 13.8-14.2 With engine running hold a screw driver to the back of the alternator, if there is no magnetic field, the alternator is bad.
If your talking about cars... If you smell a burning smell, or your red battery light comes on then i would get your alternator checked out!
You can have it tested at a auto pars store like autoZone. Another symptom is that if it has power issues, its the alternator I have the 03 LS and it needs a new alternator.
advanced auto parts stores can check it for you
A warning light on the dash would illuminate.
bad alternator, bad voltage regulator, bad battery, bad connections.