Sometimes you'll find yourself at a restaurant, hotel, or reception where you cannot predict if foods have sugar or flour in them. Your waiter may have no idea what's in a given recipe. These are manufactured to test urine for glucose. We use them to test food. Then spit a tiny bit onto a test strip.
Any color change indicates the presence of sugar.
Saliva is essential to this reaction because it contains an enzyme that releases glucose from sucrose or flour in the food, permitting it to react with the chemicals in the test strip. This is how I found that one restaurant in my neighborhood uses large amounts of sugar in its bouillon while another restaurant uses none. Solid foods can also be tested this way, but you must chew them first. The lightest color on the color chain label of the test strip indicates a very low concentration of glucose. Any color paler than this may be acceptable for most foods.