On a 65 how do you tell if your altenator is the problem or your regulator?

If you lack testing equipment, just (properly)remove the alternator and take it to a well equiped parts store. They can bench test it for you, usually free of charge. While you are at it, you may as well change the regulator- they're cheap. If you are not concerned about being concours correct, install an electronic (solid state) replacement regualtor. They are more reliable.