application_controller.rb

# One thing to add:
# * action_has_layout? method (if you have one, combine them)
# 
class ApplicationController < ActionController::Base
  # ...
    
  private
  
  def action_has_layout?
    !request.xhr? && super
  end

  # ...
end

my_controller.rb

# A long time ago, in an action far far away
def index
  # ...
  respond_to do |wants|
    wants.inline {
      # ideally use your own view (index.inline.erb)
      render :text => 'success!'
    }
  end
end