If your local shop will be the first and only sales venue for a while, ask them what's the most popular in their store or what they have ordered for quick upcoming delivery.
If you are going national and do not yet have a middle man to merchandise for you, check out the well-known fabric manufacturers for their most recent or upcoming collections. You'll learn the "new" colors and designs that are coming out. And because it will be a while before your pattern gets into shops, you'll be most au courant that way.
Jan in VA