Alternator Charging Help...

Last year about this time I had to replaced the alternator in my 71 Duster because the battery was not charging and the factory alternator gauge showed it was not as well. I had both the battery and the alternator checked and the alternator showed to be bad. After replacing the alternator with a “new one” the alternator gauge show it was charging.

I just went out today to get my Duster ready for a small High School car show tomorrow and found the battery to be very low (cranking show) and the factory alternator gauge showing the alternator not charging again.

Any suggestions, can I use a meter to check the charging out-put of the alternator and if so on what bolts/contacts do I check and what should the display read?

Thanks For Any Help!