The Health Sciences Library at the University of North Carolina at Chapel Hill has posted an easy and straight-forward tool to help authors determine if their articles must comply with the NIH Public Access Policy. Answer the questions that pertain to the article by selecting a radio button and within three clicks, authors will know "yes" or "no."
NIH Public Access Policy Toolkit http://www.hsl.unc.edu/Collections/NIHToolkit/amiaffected.cfm