Disclosed herein are methods and tests for diagnosing and/or monitoring a metabolic condition such as diabetes in a subject, wherein the methods and tests measure salivary glycoproteins. Some of the methods are based on the oxidation of glycoproteins in a sample from the subject, such as saliva or urine, for example using sodium metaperiodate, and then detecting the aldehydes generated during oxidation using a chemical detection method. Also disclosed are kits and lateral flow devices for detecting glycoproteins in a saliva sample.