S&W does not have markings like an engraved Colt revolver and the style of engraving varies with the factory engraver performing the work. S&W has three levels of coverage, namely, A, B, and C (A is full coverage and C is the least coverage). Almost without exception, factory engraved revolvers, particularly N and K frames, will have the small S&W logo roll marked on the left side of the frame under the cylinder release latch.
Beginning in the late 1970s, S&W, in order to keep up with demand, contracted engraving to a firm in Italy. While nicely done, these guns each a similar style of engraving that is very recognizable once you have examined a couple. These are factory engraved guns and letter as such, but are not valued as highly as those engraved by artisans actually working at the factory.
If you are offered an engraved gun that is represented as factory engraved, first look for the small logo on the left side of the frame, and second, ask if the gun has a factory letter verifying it was shipped as an engraved firearm. If it does not, only purchase it with the provision that it "letters".
Bill