The majority of artists do sign their paintings, some on the front of the work, others on the back. Occasionally artists sign their works in nontraditional ways, using a stamp or a symbol.
If an artwork is not signed, it is important that it be accompanied by a certificate of authenticity. An additional step is to request an Independent Party Authenticity Endorsement, which can further confirm the artwork's authenticity.
PicassoMio provides certificates of authenticity for all artworks, signed and unsigned.