Certified baseball autographs are, next to obtaining them yourself, the closest guarantee to authentic that you can get. Baseball card companies such as Topps and Upper Deck and trusted certifiers. PSA/DNA is the most well trusted third party certifier out there. MLB and Steiner are the leaders in first party certification. There are many other certifying companies, including small shops, however they are much less reputable since anyone can print up a "certificate of authenticity." If you have any questions about authenticity or certifiers, please feel free to contact me and I will do my best to give you an informed opinion on the certifier or common red flags that help identify unauthentic autographs.
The certified items I have are typically certified by the card company, although I also have quite a few MLB authenticated items. Photos can be found in my photobucket.