Diabetes mellitus can be detected by fasting blood glucose, oral glucose tolerance test, glycosylated hemoglobin, and islet B-cell function. The diagnosis of diabetes mellitus is based on the typical symptoms of “three more and one less” (excessive drinking, excessive urination, excessive eating, and weight loss) coupled with the measurement of intravenous plasma glucose ≥11.1 mmol/L, fasting blood glucose ≥7.0 mmol/L, or 2-hour glucose ≥11.1 mmol/L after glucose tolerance test at any time of the day, which can be diagnosed as diabetes mellitus. One of them is enough for a diagnosis of diabetes mellitus. The China Type 2 Diabetes 2020 Prevention and Control Guidelines include glycated hemoglobin in the diagnostic criteria, and glycated hemoglobin ≥6.5% can be used as an additional diagnostic criterion for diabetes, but diabetes cannot be ruled out at less than 6.5%. Pancreatic B-cell function tests, including the C-peptide release test and insulin release test, help determine the type of diabetes. It is recommended to visit the hospital in case of abnormal diabetes-related test results.