now only excerpted articles show the "read on" link. Also the text for that link can be set in the _config.yml

plugins/octopress_filters.rb

@@ -12,6 +12,11 @@ module OctopressFilters
     end
   end
 
+  # Checks for excerpts (helpful for template conditionals)
+  def has_excerpt(input)
+    input =~ /<!--\s*more\s*-->/i ? true : false
+  end
+
   # Summary is used on the Archive pages to return the first block of content from a post.
   def summary(input)
     if input.index(/\n\n/)