first off, what do you mean by authentic? Do you mean an early carved piece as opposed to a modern carved piece or do you mean a hand carved coin versus a die struck piece? Just like most areas of coin collecting, education is the place to start. Join the OHNS or at least go through their web site (hobonickels.org) and study the pictures. Most old pieces were done on coins dated before 1920, most modern pieces are done on coins from the 1930's. there is a OHNS group on facebook as well, but most of it is new carvers making skulls on low grade coins with the occasional high quality modern carving or early carving. there are also a lot of collectors who do have the old originals there and if you ask, they may have some for sale. If you join the FB group, mention my name in your application, I am an admin there.
Joe Paonessa
America's greatest left handed hobo nickel carver...so far