Is there a way to tell if nickel is after market?
The hammer and trigger finish on modern S&W revolvers should be color case or MIM. Nickel in these areas is far and away the easiest way to spot a refinish; experienced collectors will see it literally yards away.
The ejector rod and ejector star should be blued, and the timing pawls on the ejector star should be natural metal. These parts were never nickel finished at the factory. If the revolver has seen heavy use, the ejector rod finish may wear off, but it should still look different than the nickel on the rest of the gun.
Another hallmark is lettering and/or S&W logos with rounded rather than sharp edges, and screw holes with a "dished" or slightly counterbored look around the screws. On some older models with visible exterior pivot pin ends for the lockwork, the ends of the pins should be rounded rather than flattened, but most models made after ~1915 do not have visible external pins. These signs are caused by heavy-handed polishing prior to applying the new nickel.
Most post-WWII nickel guns have a small capital letter "N" stamped on the cylinder face and a larger "N" under the grips, normally at the LH front corner, but this varies. The absence of the cylinder "N" may indicate a refinish on an original-nickel gun because the marking is very shallow and correspondingly easy to obliterate while polishing. However, the factory was not 100% consistent about applying the "N" markings.
A star on the butt and/or the inner grip frame usually indicates a factory refinish. In my experience, S&W factory nickel refinish jobs are usually outstanding, and are not readily distinguishable from original finish using the signs summarized above. Most collectors will pay more for factory refinished guns than other refinished guns, but they will still command lower values than original finish in equal condition, and should never command a LNIB price no matter how nice they look.
FWIW if you are paying top dollar for a collector-grade gun, most S&W collectors do not consider it unreasonable to remove the grips, since rust may be hidden here and a buyer may want to verify the grip frame markings. However, some sellers may not want to do this unless they believe the buyer is really serious, and original older-style hard rubber or gutta percha grips may disintegrate during removal, so it is inadvisable to remove these grips in most cases. OTOH if a seller absolutely refuses to remove wood
stocks after seeing a legit cash offer, IMHO it is not unreasonable to negotiate a lower price.
On some guns that are very rare with original nickel- early Model 17s and commercial M1917 variants spring to mind- many serious collectors will not pay collector-grade prices without a factory letter proving original nickel finish.
[EDIT] This is my 4,000th post! Go me!