How do you know if you have a bad altinator?

If it makes a loud whining, screaming, or grinding noise, it needs to be replaced. Before you do, verify that it is indeed your alternator and not a pulley or other engine component If you suspect low output is why it's bad, the only way to verify that your alternator is bad is to attach a diagnostics tool, or remove it and take it to a local automotive parts store. Most will bench test your alternator for free.