If screeching usually starts as you take off after a car start. The alternator is working harder. Usually then just a belt slipping issue. The normal test is grab the fins that cool the alternator. Just behind the pulley. If you can turn the pulley against the belt. The belt is too loose or worn.

In those rare cases where one is not sure. Rub a soap bar along the V portion of the belt. This will increase the belt traction for long enough to verify if it is a belt or something else.

Belt tension periodic needing adjusting for wear and stretching on cars went the way of history. With the advent a long time ago now of the automatic tensioners on the serpentine belts.