Jay Recher writes:

Well, back in the day, coin scales incorporated a number of ways to detect counterfeits. Some had slots to fit the coins in as a dimension check, and of course the weight had to match specifications as well. See lots 246-258 in the Stacks-Bowers 2012 Baltimore Auction later this month. Here's one example:

To read the complete lot description, see:Lot #258. Circa 1882 American Rocker Style Coin Scale By Fairbanks & Co.(stacksbowers.com/Auctions/AuctionLot.aspx?LotID=410412)

As for books, ones I often refer to are the Standard Catalog of United States Altered and Counterfeit Coins by Virgil Hancock and Larry Spanbauer, and Counterfeit, Mis-Struck and Unofficial U.S. Coins by Don Taxay. Both are quite old now, but the content is still relevant. A more recent title is Numismatic Forgery by Charles Larson, which describes how crooks make high-quality forgeries.

The flood of Chinese copies today makes the job even harder, but many of these are relatively easy to detect. I would say the best guide is not a book or tool but first-hand experience seeing both genuine coins and fakes. Go to coin shows and look at a lot of coins. Don't be afraid to ask dealers for help learning about counterfeits. If they're not busy, many would be glad to help.

Readers: What advice would you offer Jay?