More established artists and especially dead artists usually have their authentic works catalogued and photographed in widely available reference books called catalogue raisonnes. These references are usually noted in descriptions of the works offered for sale by legitimate galleries and knowledgeable private sellers. If the seller can't provide you with these references there may be a reason. We have most of the important references and use them extensively in preparing our catalogues. Younger artists usually can be contacted directly or through their galleries for assurances of authenticity.
Many galleries provide detailed certificates of authenticity for the artists that they represent.
These certificates should pass with changes in ownership to the new owners. Print publishers can be contacted directly for information about works that they have published. This documentation is very important when purchasing expensive works. Demand a written and detailed guarantee of authenticity. Many sellers provide a simple statement on a letterhead or even a statement on an elaborate decorative certificate but providing a bare mimimun of data and even this may be inaccurate.