formats available for controller


I was wondering if there is any way to retrieve the list of formats
defined in the respond_to block of a controller?

For example if the following is in a controller:

    respond_to do |format|
      format.xml { render :xml => @people.to_xml }

Is there a way to find out HTML and XML are available? I want to write
a method that creates an XML list of all formats available for any
given controller.