Certified GKH Gardening Expert
This article includes a method of testing drainage in a soil:
This article and the Soil Triangle explain how to determine the name of your soil type based on sand, silt, and clay content. The best soils for drainage are those with "loam" in the name, so you could adjust your mix accordingly. But adding organic matter also helps a lot.
Clay is more "powerful" than sand in determining drainage qualities. Just 55% clay in a soil is enough to label it "clay" which would indicate poor drainage unless a lot of organic matter is added. You would have to have over 85% sand for a soil to be considered "sand", which is excessively well drained.