The Samuelson clinic has put together what  looks like a useful, thorough new handbook to help you determine if a work is in the public domain. http://www.law.berkeley.edu/files/Final_PublicDomain_Handbook.pdf

Most helpful is the complete FLOW CHART. We’ll put both the handbook and the flowchart in our CHARTS AND TOOLS section for your hand reference. http://www.law.berkeley.edu/files/Final_PublicDomain_Flowcharts(6).pdf