Get the model number and look it up. Somewhere on that alternator are numbers, and they'll tell you what you have. If it was rebuilt by itinerant jokers who ground off all the numbers...you can still open it up and look inside to see if there's a regulator. Or, ask someplace that sells and services alterantors what you've got.
And old used anonymous alternator wouldn't seem too reliable to start with, even compared to buying a cheap one from a junkyard.