A drop of potassium iodide on food will turn dark in the presence of starch. The best way to actually quantify the amount of starch in food is by reading food labels for carbohydrates and doing some math.
No, I would say using a centrifuge and finding out how much the starch content is. But, say you got potatos boiled them to make them soft blended and got potato puree, putit in a centrifuge you could get an estimate of how much starch is in a potato based on size. Why do you ask?
Reading labels is not going to be easy: they give total carbohydrate and sugar, the difference should be starch. However, these are not accurate figures. For most accuracy look in reference books such as Food Values of Portions Commonly Used; a standard text for nutritionists and food scientists