Mule Design’s Mike Monteiro wrote a cracking guide on how clients can give better feedback to designers.
Let the design team be the design experts. Your job is to be the business expert. Ask them how their design solutions meet your business goals. If you trust your design team, and they can explain how their recommendations map to those goals, you’re fine. If you neither trust them, nor can they defend their choices it’s time to get a new design team.
This should be printed out and nailed into the forehead of every designer and their clients a la Luther’s Ninety-Five Theses, you know, for easy reference.