How do you learn to recognize a fake hatpin when you see one?  By practice and by doing comprehensive research before buying.  It is important to learn about the construction characteristics that authentic hatpins share.  While not all hatpins are made in the same manner, they are very similar in important ways!

First off, learn about findings.  Findings are the small finishing pieces that cover the connection of the pinstem to the ornament.  Findings are varied in style but still they are specific to the hatpin era.  Plain and simple, the best way to learn about findings is to study period hatpins.  The small Lillian Baker book has drawings of finding styles found on hatpins but that is still no substitute for seeing them first-hand.  All of the Baker books are great to have to see the lovely photos of authentic hatpins to give you an idea of the styles of the era but more so in that you can see the original findings used on those hatpins!  One note is that you shouldn't ever buy a hatpin on ebay without seeing the finding as it is a first clue to a hatpin being a fake.  If you see any type of sodder, glue or other adhesive then it is recommended that you assume the hatpin is a fake as authentic hatpins were not constructed in this manner.  Yes, sodder on the back of a hatpin could mean a sloppy repair job but more likely the ornament was not originally part of a hatpin.  Findings should also be centered on the back of a hatpin.  If the design of the hatpin does not make a centered finding applied directly to the back of the piece appropriate (such as heavy reposse' work that does not give a good foundation for the attachment of the finding) then a bridge or arc-type finding would have been used.  Off-centered findings are a clue to a hatpin being a fake and are often seen on the reproduction hatpins known as the Stamped Metal Hatpins to experienced collectors.  These fake/reproductions are often seen on ebay in styles such as a horse with red rhinestone accents that are glued in and many, many more both with and without the rhinestone accents.  Memorize the style of the finding used on these stamped metal hatpins too as they are not authentic to the era.  They are similar enough  to authentic findings though to fool collectors who don't know their findings.  If you see a bridge or arc-type finding then make certain that it fits the back of the hatpin.  The edges of the bridge/arc should never overlap the edge so as to be seen from the front of the hatpin.  Beware of hatpin ornaments with a metal backing that appears to have a hole punched in the metal for the pinstem to be inserted; this is most likely a former button that someone has made into a hatpin to fool collectors.  This of late has been the case with a few Satsuma-style hatpins.  The exception to this rule are the often home-made hatpins that feature military or other service clothing buttons.  During the war years it was common to honor and remember a loved one who was away by wearing a button from their uniform and this was often done via hatpin!  Learn the difference between a tubular, patch, bridge, arc findings, etc. and you will be taking a giant step toward avoiding fake hatpins.  Nodder or trembler hatpins have their own specific findings along with two common styles of springs used.  One spring is a simple one that is less than an inch long and all of one size.  The second is a cone-shaped spring (harder to find) often with a tiny chain in the center that secures the nodder's ornament to the pinstem.  There is a seller on ebay who of late consistently offers what he believes are incredible nodder/trembler hatipns but they are not old and should not be confused with antique nor vintage hatpins.  A typical fake finding often seen is shaped like a tiny funnel and this is exactly the type used on the fake nodder/trembler hatpins.  If you want a quality collection then avoid all hatpins made-up with this finding!  Also, the color of the finding should match the coloring of the ornaments setting.  A silver hatpin with a brass finding should raise flags of doubt. 

Secondly learn how stones were set in period hatpins.  Stones were seldom glued or pasted onto a hatpin.  There are exceptions to this rule such as the tiny rhinestone accents used for eyes on the sterling teddy bear hatpins.  Stones should be properly set in various ways such as claw, bezel, crown, sleeve, box, channel, illusion, etc. It has become rather common on ebay to see newly made-up  Czech hatpins with large stones faceted on one-side only that are set on-end with a metal filligree piece that is folded around the stone as a setting.  Often a second stone will be glued (rather than properly set) on one-side as an accent.  If the hatpin, when worn in a hat could be seen from two sides then both sides would match. One side wouldn't be finished and the other side not.  AuthenticVictorian and Edwardian hatpins were NEVER constructed in this manner.  Stones set on end can be seen in original hatpins but they would never be faceted on one-side only nor set in the manner that these common fakes are done.  A note on the handpainted or transferware porcelain disc hatpins: the disc should fit the setting without the disc over-hanging it so as to leave the delicate porcelain edges at risk of damage.  If the prongs of a setting are stretched out so far as to barely hold a porcelain disc, gem or other item then that should raise a red flag warning telling you that the item probably isn't original to the setting. Also, findings were never simply glued on the back of the porcelain discs sans proper setting either.  

As a general rule, hatpins made of precious metals should be marked as to the gold or silver content but there are of course some exceptions to this rule as hatpins were made all over the world and each country had different requirements for marking these pieces.  One good thing to look for on these highly desired hatpins however is pinstems that match the ornament in color just as the finding should.   

Another thing to look for are ornaments that feature things of the period.  Kewpies, Billikens, and other characters were quite popular during the hatpin era and would appropriately be found as hatpin ornaments.  Flappers are another matter as they came in the era following the end of the long-pinstemmed hatpin era.  Why then should you find a flapper on a long-pinstemmed hatpin?  To put it plainly, you shouldn't!!!   (Some have termed the Flapper hatpin as a Dolly Dingle but one look at a drawing of the Dolly character clearly shows that this is not the case.)  Flapper hatpins are out-and-out FAKES.  They make a nice conversation piece but they certainly aren't authentic as long-pinstemmed hatpin ornaments. 

One should realize that while some hatpins are indeed rare as in the compact hatpins,  not all  Victorian and Edwardian hatpins are.  Authentic and quality hatpins however are still hard to find.  With that, sellers who offer the exact same style of hatpins over and over again should be suspect.  An excellent example are ivory hatpins.  These are especially hard to find and yet one seller has a number of them nearly every week on ebay.  Ditto the bronze-colored stamped metal hatpins that have been around since the early 1980s and initially fooled even the best of collectors.  Even simply made glass hatpins should be suspect if you see the very same ones over and over and over again by the same seller.  If you see a lot of hatpins that are similar from one seller, then you should be wary!  Never  simply rely on the auction title or description that a seller offers.  They might believe that their hatpins are authentic, Victorian, Edwardian, antique, vintage, awesome, jaw-dropping, or whatever.  But that doesn't necessarily mean that it is true.  More often than not it isn't!  A good rule of thumb to remember is that even on a good day, sadly over 90% of the hatpins listed on ebay are fakes!

Last but not least is where to go to learn more about hatpins?  The Lillian Baker books are required reading, hopefully even before one begins to collect.  They are often found on ebay at auction, even the out-of-print pink and blue covered edition of  "The Collector's Encyclopedia of Hatpins and Hatpin Holders" will occasionally be found on ebay.  Other resources include the American Hatpin Society or the Hatpin Society of Great Britain.  Both offer memberships that include quarterly newsletters and both also feature websites with some great photos and information.  Experienced collectors are another great source and they are usually very happy to help answer questions about hatpins.  (If  you are bidding on an ebay auction however, ask BEFORE bidding!) 

Good luck hatpin hunting!  It's a jungle out there!