You don't have to seek these Cassndras out; if you are in management, they will find you. Like somebody who sells a product that he is passionate about, they will "sell" their concern to you with a passion. Don't argue with them; even though it's time-consuming, do your best to hear them out, to learn what they know and to understand why it affects them the way it does.
Mark Twain hit it on the head when he said, "Put all of your eggs in one basket and watch that basket."