In a perfect world you would have the exact reference of the alternator or starter ! If you do not have it, maybe the faulty part has one !
In that case, insepct the part thoroughly and try to find the part number printed on it.
Grab a magnifying glass if needed.
In few cases there might be NO reference on the part, if you do not have the reference of the alternator or starter, it is not possible to know which parts are inside.