For the most part you will need to fly to each belt and hope that you strike it lucky. However, here is a general guide that will help you know which belts to look in.
When considering which ore to mine, make sure you look at its price per volume, not price per unit. Effort in mining and hauling are both limited by volume. Scordite might sell for twice as much as veldspar, but it takes 1.5x the effort to mine it. Therefore, it is still more profitable to mine, but not a full 2x.