Is it the Alt. or Volt. Reg. ?

If your alt is a dual field (Three wires connected to it) ground one of the field wires to the alt case and if the alt is ok it will go to full charging output.
The easy way to tell is that it will pull the engine idle down, and the whine from the alt will get louder.
Don't rev it while doing that though, cause you could blow something from overvoltage.

Just ground a field terminal to the alt case and see if the idle drops.

If it does then it's the regulator.
If not, then it's the alt.