How do you get a Certificate of Authenticity for an autographed item that confirms the autograph is real?

Two of the top, and most trusted authenticators are PSA/DNA, and James Spence Authentic (JSA) Collectors will pay more for signatures authenticated by either one. They both do business through the mail, and also travel across the country. Check their websites for their show schedule for when they might be in your area. Before you spend money be sure that they are not facsimile signatures. A lot of people mistaken facsimile signature which are printed on, as opposed to authentic hand signed signatures. If the signatures are signed with a felt pen for example, the ink will tend to be heavier at points where the pen started, stopped or slowed while signing. With facsimile signatures the ink will trend to be more uniform throughout. I will leave a link on this page to read more.