Authentication is only performed by SC's, and I believe only by a very small group qualified to make such authentications.
For what it's worth, there is no known rep spotter's guides, or bibles, per se that Rolex S.A. issues out. Rather, there are archives and records of serial numbers (case and movement), reference numbers, identified regions and/or specific dealer locations that the watches were shipped to, etc. This is why some SC's can make certain authentications, while others cannot. A good example is the Mil Sub (5517)- Rolex UK is qualified to make such an authentication as the Mil Subs were specific only to that region, and Rolex UK would have the history on those watches as that is where they originated from. Rolex NYC could not make an authentication on such a piece as they have no information on these specific watches. The authentication process itself is performed by an in-house specialist- Cross checking of serial numbers, reference numbers, examination of parts all takes place by a small group of people within the SC. Between the referencing of records as well as physical examination of the watch, authentication can be given. And, yes- I believe that those responsible for authentication can provide proper authentication of parts by examining tell tale characteristics that cannot be duplicated- Things like font, dial printing methods, dial bases, etc. As good as some replications are, they simply cannot duplicate certain characteristics, unless the same methods were used to produce the replication- And THAT is what is a heavily guarded secret at Rolex. Besides... Handle enough Rolex watches day in-day out for years of your life, and you develop not only an eye for such things, but also a feel.
R