The color dots in the selvage are a good starting point. For me, the trick is finding fabrics that blend with each other, rather than perfectly match. Vary the scale (pattern size) as well. When in doubt, you can always add in some tone-on-tone fabrics in other colors of your main pattern. I use a lot of marbles and Kaufman Fusions in my quilts.
Looking at collections, you will see how they usually vary in color and scale. Some manufacturers use similar colors in their collections, so you can often find something by looking at the same mfg. in a different line.