Alternator discharging, bad alternator?

We actually drove this 65 valiant 170 down the road yesterday. It was having stumbling issues. A mechanic came and adjusted the points and reinstalled the distributor. Then advanced the timing. It did pretty good down the road we only raised it up to 45-50 mph because it was a neighborhood. But he said the alternator was charging but not enough. Is that a bad alternator or regulator? How do I know?