The best way to identify an artist's signature is to have an intimate understanding of the monograms, signatures or other markings common to that artist's work. Failing that, one can consult either a free or paid online database that offers examples of various artists' signatures.
A foolproof way to identify an artist's signature is to know his work and style very well, including any trademark markings he commonly employs. A good forger, however, is not only able to duplicate the style and aesthetic of any renowned visual artist, but he is also able to reproduce that artist's signature with perfect or near perfect fidelity.
In the case of artists such as Jackson Pollock, whose work has been the source of a few notorious forgeries, forensic art specialists often search for the artist's finger prints embedded in the paint of his canvases, which offers a foolproof way to guarantee their authenticity. For most artist's works, however, this is not an option.
While knowing the manner in which an artist signs their work is helpful, in terms of verifying its authenticity, it is more important to know the provenance of the piece in question. The chain of ownership can indisputably link the work back to the artist who created it.