There are several qualitative tests for food chemicals. These can be used to detect the presence of food chemicals, but not how much is present.
Benedict's test is used to detect sugars.
Sugars classed as reducing sugars will react with Benedict's solution on heating for a few minutes. Glucose is an example of a reducing sugar.
Reducing sugars give a red-brown precipitate with Benedict's solution. The precipitate takes a while to settle in the tube - you're more likely to see simply a red or brown colour. If there's not much glucose present, the final colour may be green or yellow, or orange if there's a little more.
Add iodine solution to the food being tested.
Foods containing starch will turn a blue-black colour.
The iodine test can also be used with a microscope to stain starch grains in plant cells.
The biuret test is used to detect peptide bonds in proteins.
Add Biuret solution A to a solution of the food being tested and mix carefully. Then trickle a little Biuret solution B down the side of the tube. Look for a purple colouration where the solutions meet.
Biuret reagent is sometimes available as a single solution.
The Sudan III test is one test used to test detect lipids.
The emulsion test is an alternative test for lipids.