Rails 4: check if a local variable exists and is true in a partial

You can check the local_assigns hash, which includes the passed locals.

<section 
  id="view-dashboard" 
  class="<%= 'embedded' if local_assigns[:is_embedded] %>"
>

when rending the partial do this
<%= render partial: "some_partial", locals: {is_embedded: true} %>

#_some_partial.html.erb
<%
 #locals
 is_embedded ||= false
%>
<section
id="view-dashboard"
class="<%= "embedded" if is_embedded %>">hhd</section>