This is the big unanswered question about cloud computing. The reality is that most organizations are not evaluating their cloud providers at all. That’s a dangerous approach. You absolutely need to be asking the tough questions up front and then making sure they keep their promises moving forward.
Periodic and consistent is the best approach – whatever that means in your environment.
In the interest of adding some humor to this discussion while driving home the point this is a must-see video on cloud computing security oversights.