If you're a consultant/outside entity being asked to do this, then no worries. Just tell them like it is. You don't need any kind of technical test here: Just show them the trail of failed projects and unhappy employees/customers and how they all lead back to one source.
But if you are part of the company...or worse part of IT yourself....watch the fuck out.
Something stinks about this. Managers are usually the first target when it comes to determining blame for failed projects/bad internal PR. That's part of their job after all. The stuffed shirts know this, and if it was just about the IT Manager being terrible they'd fire him and bring in someone new. They don't need a third opinion to tell them the head of IT is incompetent. There's something else going on here, probably related to internal company politics, and you need to be sure you're not being thrown under the bus or are risking getting caught in the crossfire.