1992 Toyota Previa Battery or Alternator?

This is a Japanese-market turbo-diesel van. At idle, accessories, wipers, etc. are very slow, and speed up noticably at higher engine speeds (although the wipers still have trouble keeping up with Vancouver rain!). Is this most likely a weak battery issue, or a charging problem? The van has no trouble starting, and I haven't heard any belt noise. No warning lights have come on. Maybe I'm being too picky, but I want to sort everything out before the cold weather sets in!
Thanks for your help!