You can find the full source code for this website in the Seam package in the directory /examples/wiki. It is licensed under the LGPL.
This example gets the 'facelets.DEVELOPMENT' init parameter.
#{facesContext.externalContext.getInitParameter('facelets.DEVELOPMENT')}
But in order to keep from typing so much you can declare it in a factory.
<factory name="isFaceletDevelopment" auto-create="true" value="#{facesContext.externalContext.getInitParameter('facelets.DEVELOPMENT')}"/>
now that you have done so you can merely reference it as
#{isFaceletDevelopment}
and plug that in into any bean or page.