For an exact answer, take a sample of your current soil in to a lab and have a soil particle test done. This test can give you particle sizing and exact percentages of the sand, silt and clay in your current infield mix. Contact a Ewing Sports Fields specialist for more information.
For a rough estimation of the soil types in your infield mix, try the jar test. Simply fill a glass jar ¾ full with your current infield mix, and then fill the jar the rest of the way with water. Give the jar a good shake and then set it down and watch. The sand, silt and clay will separate out, and you will be able to get a rough estimate of how much of each soil type is in your current infield mix.