Chemical reagents let you identify the nutrients in a food by a colour change. First prepare the sample: crush the food, mix it with distilled water and filter it, so you test a liquid. Always run a control with distilled water, so you know what a negative result looks like.
Starch: add a few drops of iodine solution. It stays orange-brown if there is no starch, and turns blue-black if starch is present. Protein: add biuret reagent. It stays blue with no protein, and turns purple (lilac) if protein is present.
Reducing sugars: add Benedict's reagent, which is blue, then warm the tube in a water bath at about 80 °C for a few minutes. Heating is essential. With no reducing sugar it stays blue. A small amount gives green, more gives yellow or orange, and a large amount gives a brick-red precipitate, so the test is semi-quantitative.
Fats: shake the food with ethanol, then pour the clear liquid into water. Fats dissolve in ethanol but not in water, so if fats are present they form tiny droplets and the water turns cloudy white. This cloudy mixture is called an emulsion. Wear eye protection throughout, keep ethanol away from flames and take care with hot water.