@ The Silver Searcher .... As Cointagous points out, comparing c/s pics, thereby comparinging font styles, measurements, alignment, spacing and such with known specimens, is the best way to assess genuine issues. You can best find pics for comparison c/s's at major auction archives, like Heritage and Stack's. Reference books by Brunk & Rulau are must-haves, methinks.
I've written many dozens of articles on c/s's for the National Token Association, the Civil War Token Society and a variety of other numismatic publications. The Sage c/s article that Cointagous mentioned was one of mine. One of the reasons for my writing about c/s's is to get pics published, so collectors can see what these rarely issues look like. Also, you can see many c/s's on the CCF threads, some of which I've posted.
To my knowledge, NGC and other
TPG's don't authenticate c/s's. If a c/s'd coin/token is slabbed, their authentication only pertains to the coin/token. That said, I've rarely encountered a faked c/s. One that immediately comes to mind was a crudely misaligned replication of a
VOTE THE LAND FREE c/s. As a rule of sorts, I generally avoid buying c/s's that were created with individual letter punches, resulting in misalignment as such.