Look for signs that the same amount of pressure was used throughout the signature. If the letters look perfectly even, this is a sign that an autopen or mechanical arm was used to create the autograph.
Look for signs of vibrations in the letters of the autograph. These are not the same as the normal shakiness a person might experience when signing his name; vibrations may cause fuzziness or slight misplacing of letters that indicate the signature was made with an autopen.
Compare the signature to other versions of the celebrity's signature. This will help you to determine a harder-to-detect form of forgery: signature by the celebrity's secretary. Since this is authentic human writing, and the secretary has probably learned to mimic the celebrity's writing very well, you will have to look for subtleties that make the celebrity's writing unique and see if they are present in both signatures.
Track the item's history to its original source. This, while the most difficult and time-consuming method of verification, is perhaps the best way to determine authenticity.