I 'accidentally' learned one sure way to tell if the 'shrieking' or 'squealing' is caused by a faulty bearing and which accessory is the culprit.
My car had been briefly 'squealing' off and on for a couple weeks. One night as I'm driving to work it was really loud and didn't stop like it usually did. So I stopped at a gas station to look the engine over. When I raised the hood, I looked around a little, didn't see anything, so I leaned in for a closer look, placing my hand on the top of the alternator to support my weight. Woohoo!
The alternator was extremely hot. Not just 'extremely warm' as alternators get under normal circumstances, but burning hot. Good thing that area of my palm was protected by callous, it glazed the dead layers of skin and resulted in a minor burn. But there was a nice 'sizzle' sound when I touched it. lol!
So I replaced the alternator the next day and all was well. Note that I am not recommending this method of diagnosis. I said it was a 'sure' way to tell, not a safe one. :P
I was actually going to recommend holding your open palm a few inches away from the alternator or other accessories to see if there is an unusual amount of heat radiating from any particular accessory, not touch them. After I burned myself, I held my other palm about six inches away from the alternator and could feel the heat coming from it unlike any other accessory.