Fehling's test is a biochemical test used to detect aldose monosaccharides. Aldehydes gives positive fehling's test while ketones give negative fehling test. however some ketones like alpha hydroxy ketones (eg fructose) does give positive fehling reaction. During this reaction the aldehyde group is oxidised to acid while the copper ions are reduced to red/brown precipitate of copper oxide. This is a common test used to detect glucose in urine as positive indication of diabetes.