Call them
Just get the phone number from the S&W website, if you don't have it, and call them. They will tell you the year it was made. That is what I do with Smiths. Some other mfrs, besides Colt, might not be so helpful. If it was a collectable gun, they will sell you a factory letter that gives specific dates, finish, and shipping info, like a Colt factory letter, but much less expensive.
Otherwise I have a S&W book by Jinks, but it only goes to the 70's.