Fehling's solution test is used to detect the presence of reducing sugars. Carbohydrates that have a free aldehyde or ketone group (or a free hemiacetal/hemiketal group) act as reducing sugars. Glucose is an aldohexose and has a free hemiacetal group that can open up to form an aldehyde in solution, thus it readily reduces Fehling's solution (giving a red precipitate of $\text{Cu}_2\text{O}$). Sucrose is a non-reducing sugar because both of its anomeric carbons are involved in the glycosidic linkage. Fats and proteins do not give a positive Fehling's test.