Instructions
Compare the autograph to a professionally authenticated autograph of the same celebrity. Authentication services such as Professional Sports Authenticator and Beckett offer reliable authenticating services. If you can compare a genuine signature to an unauthenticated autograph, you can greatly increase your odds of making a correct determination.
Hold the autograph up to the light. If the ink of the autograph is too light or had equal pressure applied throughout the name, it is likely a fake.
Examine the autograph to see if an autopen may have been used. An autopen is a machine that uses a pen to replicate original handwriting. If a name is signed with a pen, the nib will cut through wet ink to produce "tunnels" and "bridges" that are visible through a magnifying glass. These "tunnels" and "bridges" will not occur if an auto-pen was used to create the autograph.
Run your thumb over the signature. If the signature is flat, it's likely stamped on. Fakes tend to have a flat feel to them.
Look through a magnifying glass for signs that the autograph has been stamped on. If the autograph is too smooth and perfect, it is likely a fake. Human signatures contain imperfections.