I'm assuming you're asking how to test for a starch,
To do so you do this thing called the "Iodine Test"
The potassium iodide aqueous solution will form this complex with starch to form a purplish-blue color.
However, watch out for the pH as hydrolysis of starch could occur to prevent the test from working out-- and the temperature as the greater it is, the less intense the color is
