A Catalogue Raisonné is a compilation of the “complete works” or “oeuvre” of an artist and can be located by searching OskiCat, e.g. Whistler raisonne. They can be used to identify paintings, or at least to offer stylistic and technical comparison points. They are particularly useful for prints, as they identify, describe and usually illustrate each state of a print, making precise identification easier. They also can include edition size, paper and publication information useful for authenticating your print.
In the case of an unsigned painting, possibly by a noted artist, it’s often necessary to call upon an expert. A person specializing in the work of a particular artist, or in a particular genre of art can quickly authenticate or spot inconsistencies in a work. Appraisers charge a fee for their services.
Guide International des Experts et Spécialistes
This directory lists experts (most located in Europe) and their contact information in order of the artist in which they specialize, as well as by individual’s name.
IFAR offers an authentication research service
Print Council of America
Search by artist name to see if a catalogue raisonnés exists for a particular artist, specifically for their prints.
Achenbach Graphic Arts Council, Fine Arts Museums of San Francisco
The Achenbach provides much of its collection online, and makes almost any work of art available for viewing by appointment. This service is particularly useful if, say, you have a purported Whistler print, and want to view it next to a comparable Whistler print in their collection.
Authors of catalogue raisonnés or artist monographs may be willing to offer an opinion on an important artwork, particularly if they’re in looking for works to include in a publication. They may be contacted through a publisher, and, if they are active professors, sometimes their contact information is available online, through the websites of the universities at which they teach.
As mentioned above, auction house appraisers can often help with research, although this usually requires that you enter into an agreement to sell your art with this gallery.
Some museum curators are willing to point researchers in the right direction, particularly if they themselves are experts in that specific field.
Some gallery owners are willing to offer a casual opinion as to the authenticity of a work. Keep in mind, though, that if they are interested in purchasing your artwork and selling it in their gallery, they stand to benefit if you are under the impression that your art is of low-value.