To find examples of famous artists signatures, you can consult with professional art appraisers. If you already have examples of signatures, search websites such as ArtitstsSignatures.com and IdentifyArtistSignatures.com.
A good place to start looking for examples of famous artists signatures is your local library. Books on art, especially those that focus on particular artists, often give examples of the different signatures that have been verified by art experts.
In addition, there are books of verified exemplars published specifically to help art owners and dealers authenticate signatures. The Getty Research Institute recommends texts such as Artists as Illustrators: An International Directory with Signatures and Monograms, 1800-Present and Encyclopedia of Artists' Signatures, Symbols & Monograms: Old Masters to Modern, North American & European plus More; 25,000 Examples.
As you learn more about verifying artists signatures, you can use online services such as Artists Signatures and Identify Artists Signatures to add your collection of exemplars. However, before pricing a piece of art for sale, for example, you should have an expert verify a signature. You can hire a professional art expert, or pay for an online verification. Because authentication is expensive, check to see if the piece is already insured because an insurance appraisal includes authentication.