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.