<p>I want to do a conditional rendering at the layout level based on the actual template has defined <code>content_for(:an__area)</code>, any idea how to get this done?</p>

Ok I am going to shamelessly do a self reply as no one has answered and I have already found the answer :) 
Define this as a helper method either in application_helper.rb or anywhere you found convenient.
```
```
def content_defined?(symbol)
    content_var_name="@content_for_" + 
      if symbol.kind_of? Symbol 
        symbol.to_s
      elsif symbol.kind_of? String
        symbol
      else
        raise "Parameter symbol must be string or symbol"
      end

    !instance_variable_get(content_var_name).nil?

  end
```
```