Check if yield is defined

I couldn't find a solution to this question.

Well, on my application.html.erb I defined a content space:

<%= yield :scripts %>

I have a view app.html.erb who use this space to put some contents:

<% content_for :scripts do %>
<script>
  $(document).ready(function() {
    app.init();
  });
</script>
<% end %>

I would like to know if is possible check if yield :scripts exists on application.html.erb. If not I would like to print this part of code (inside content_for) on my app.html.erb view (sometimes this view was called by ajax so I need that these code come with the view).

Something like:

if :scripts.exists?
    content_for :scripts do
        ...code...
    end
else
    ...code...
end

Thank you!!

Obs: My Rails version is 3.2.21.