I had something like that happen to my Fiero once. It has a separate belt that only drives the alternator and the AC pump (which doesn't work). That belt snapped, but the battery held up through a couple of starts before leaving me stranded an hour from home.
Most parts shops I've used have a free tester that they'll throw on your car for you to help you figure out the health of your battery, alternator, voltage regulator, etc. Canadian Tire up here will do that for you for free.
I've also found that newer car batteries seem to die an instant death. I remember having old cars years ago that you could tell for a week that the battery was going to die. Just kept cranking slower and slower every time you started the car. Now you'll just be stranded in some random parking lot with no warning.