An initializer to show what parts of your page come from what rails partials

explain_partials.rb

# Start the app with EXPLAIN_PARTIALS=true to show locations of view partials
if Rails.env.development? and ENV['EXPLAIN_PARTIALS']
  module ActionView
    class PartialRenderer
      def render_with_explanation(*args)
        rendered = render_without_explanation(*args).to_s
        # Note: We haven't figured out how to get a path when @template is nil.
        start_explanation = "\n<!-- START PARTIAL #{@template.inspect} -->\n"
        end_explanation = "\n<!-- END PARTIAL #{@template.inspect} -->\n"
        start_explanation.html_safe + rendered + end_explanation.html_safe
      end

      alias_method_chain :render, :explanation
    end
  end
end

Save this file to config/initializers/explain_partials.rb