home_controller_test.rb

# When requesting mobile you can set the request environment two ways
# @request.env['HTTP_ACCEPT'] = "mobile"
# OR
# @request.accept = "mobile"

require 'test_helper'

class HomeControllerTest < ActionController::TestCase

  test "should get mobile index" do
    @request.accept = "mobile"
    get :index
    assert_response :success
    assert_select "div[data-role=header]" do
      assert_select 'h1', 'Your Header Title'
    end
  end

end

index.mobile.erb

<div data-role="header">
  <h1>Your Header Title</h1>
</div>

mime_types.rb

Mime::Type.register_alias "text/html", :mobile