Wal-Mart has a battery charger that will also allow you to see your battery voltage and alternator voltage. They are pretty cheap. I'd be surprised if the belt could slip so much that the alternator couldn't charge your system. It might be a combo of lose belt and alternator having a problem though, or just the alternator.
It would be a pain, but you can take the alternator off and have it checked at an autoparts place. Don't but a replacement alternator from Autozone though. My stepson got 5 from there, before finally getting one from O'Reily's.