How can I tell a A cutout from a T?
Original stock type you can't because they were all the same, - or + ground, system did not care. The modern style, diode or other, different story.
I think the wire connection is a little different on a Model A, plus a Model A is positive ground.
Mark is totally correct. The reverse current relay type cutout used on the Model T and A is not polarity sensitive and fully interchangeable.
That is not the case for diode conversion cutouts and Voltage Regulators sold for Model T and A's