At the end of photosynthesis, the green leaf is tested for starch and not glucose, because

During photosynthesis, the glucose made in the leaf is quickly changed into starch for storage. So little free glucose stays in the leaf, but starch builds up.

That is why the iodine test for starch is used. It turns blue-black when starch is present. Glucose is not used up at once, and starch is stored in other parts of plants too.