Once you’ve determined exactly what you have along with provenance and its value you’ll want to ensure both that the artwork has never been stolen, and that, should it be stolen in the future, you’ll have the best possible chance of recovering it.
The AAM Guide to Provenance Research by Nancy H. Yeide. Washington, D.C.: American Association of Museums, c2001. N3999.Y45 2001 AH/C Reference. Guide for tracing the ownership history of works of art. Focused on cultural property looted by the Nazis and others during WWII, it is divided into three parts: Basic Provenance Research and Principles; Holocaust-Era Provenance Research; and Appendixes, which include bibliographies of collections, dealer archives, and “red flag names” compiled by the Office of Strategic Services.