Is there a way to test which layout an Action Mailer email is rendered with?

I have some Action Mailer email messages and I would like to test which layout will be used to render the email. I found this example on the web, but it's from 2008 and doesn't work with Rails 3.2 and presumably later versions.

My motivation for this is that I'd like to write a unit test that asserts that the mailer was rendered with a particular layout so if that is changed, the test will break.