Right, the method for earlier S&Ws would be the presence or absence of a B with the SN stamped on the barrel. Later, and I do not know exactly when, S&W stamped a capital N on the grip frame, left side, and/or a capital N on the rear face of the cylinder. It is sometimes difficult to detect non-factory nickel plating, but it usually can be done. Not unusual that a non-factory plating job will include plating the hammer, trigger, and sometimes the extractor star, which would not have been plated by S&W.
Whenever one sells anything other than in a face-to-face transaction, the seller has a duty to be absolutely certain about describing exactly what it is he is selling, especially in the areas of problems and finish originality.