Diabetes can be diagnosed in three ways. First, your doctor can measure your blood sugar levels through a blood test. If your levels are higher than 140 mg/dL (7.8 mmol/L) after fasting for 8 hours, then you have diabetes. Second, a glucose tolerance test can be used to diagnose diabetes. This test measures your body's ability to process glucose and if your levels are higher than 200 mg/dL (11.1 mmol/L) after two hours, then you have diabetes. Third, your doctor may use an A1C test to diagnose diabetes. An A1C test measures your average blood sugar levels over the past 3 months and if your levels are higher than 6.5%, then you have diabetes. [1] Additionally, if you are pregnant, your doctor may test you for gestational diabetes.