The dash board light is part of the circuit, so check that first. However this is an all or nothing thing.
Low output can be down to a blown diode in the rectifyer, but I think this is integral with the regulator, so that's not it.
So you're left with simple stuff to check, tightness of drive belt, clean and tighten connections on alternator, battery and ground straps.
If these are all OK, you could have a problem in the windings of the alternator.