How to detect if rails is at the root url?

What I want is seems simple.

In my application helper i setup this

module ApplicationHelper

def isroot
if root_url 
@container = "mainbox"
else
@container = "maincontainerbox"
end
end
end

in my application layout i have this

 <div id="<%= isroot %>">

How do I find out if my application is at the homepage. if not do something else.