Place the leaves in boiling water for six minutes to soften the cells, and then in ethanol (alcohol). When the leaves are drained of colour, spread them out on a flat surface. Soak the leaves in iodine and the green parts will turn blue/black and the non green parts will stay the yellowish brown colour of iodine liquid. The green parts contain starch, which makes iodine turn blue/black. The blue/black stain is what shows starch is present. You could take "before & after" photos to prove the black areas were green at the start.