There are so many fake signatures out there and it's difficult to know if it's an authentic signature offered in online auctions or whether it's a secretarial signature you just received in the mail.
Autograph World has an extensive guide of autographs allegedly received in-person. It can be a good source in the area of authentication. The guide contains mostly autographs of celebrities.
http://www.autographworld.com/sigs/list.asp
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