A positive test result for starch occurs when the substance in question causes a solution of iodine to turn a deep blue or purple-black color. The iodine retains its usual yellowish color when no starch is present.
According to the BBC, this color change occurs when the amylose in starch reacts with iodine. Starch is simple to test for at home, since iodine is readily available at most drugstores. A little iodine is mixed with the substance of interest; if starch is present, the color change occurs very rapidly. This test is useful for people who are starch intolerant. It can also be used by brewers to monitor the progress of a mash, the process by which starches in grain are converted to fermentable sugars.