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

.themes/classic/source/_includes/article.html

@@ -12,9 +12,12 @@
 {% endunless %}
 {% if index %}
   <div class="entry-content">{{ content | excerpt }}</div>
+  {% capture excerpted %}{{ content | has_excerpt }}{% endcapture %}
+  {% if excerpted == 'true' %}
     <footer>
       <a rel="full-article" href="{{ root_url }}{{ post.url }}">Read on &rarr;</a>
     </footer>
+  {% endif %}
 {% else %}
 <div class="entry-content">{{ content }}</div>
 {% endif %}

_config.yml

@@ -32,6 +32,7 @@ pygments: false # default python pygments have been replaced by pygments.rb
 paginate: 10          # Posts per page on the blog index
 pagination_dir: blog  # Directory base for pagination URLs eg. /blog/page/2/
 recent_posts: 5       # Posts in the sidebar Recent Posts section
+excerpt_link: "Read on →"  # "Continue reading" link text at the bottom of excerpted articles
 
 # list each of the sidebar modules you want to include, in the order you want them to appear.
 # To add custom asides, create files in /source/_includes/custom/asides/ and add them to the list like 'custom/asides/custom_aside_name.html'

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/)