Voltmeter is a good idea. It helped me with my electrical gremlins and it's cheap. I didn't notice the idler pulley on your system. Good luck.
Note: Voltmeter is just voltage and will tell you how much juice is flowing in your system. It will let you see if your system is draining juice or holding at charge while running. Amp meter is more complicated to connect and will tell the amp output of the alternator.
EDIT: I looked back through your photos and I still don't see what I'd call a serpentine belt. Maybe we have the same thing in mind with different terms. I call what you have in the photos a V belt. It just runs the alternator. Serpentine runs multiple devices:
http://marchperf.com/gallery_images/30126.jpg
Dale