What is the best way to test a person to see if they are diebetic?

Blood sugar. Blood sugar is a test for diabetes that can be measured at various times. A fasting blood sugar should be less than 100 mg/dl, and a blood sugar taken randomly should not exceed 200 mg/dl. Blood sugar 2 hours after a meal should be less than 140. If you think you might have diabetes, you should see a physician to check your blood sugar. A blood test called A1c can also be done. No fasting needed.
Hemoglobin A1C. Certain microscopic cell changes may occur first, but these are used for research. Normal bs after an 8 hour fast is 65-100 mg. 'impaired glucose tolerance' is FBS 100-126. High bs is >126, diabetes mellitus, type 2; must be done at least twice to make dx. Hgb A1c is a more accurate test, because it measures what the average sugar has been over about 6 weeks. A confirmed HGB A1c over 6 makes dx.
Blood Glucose Check. The best screening test for evaluating if a person is diabetic checks the baseline amount of glucose (sugar) circulating in someone's blood when they have fasted (no food or drink) for ~8 hours. Such tests are easily obtained at almost any doctor's office, and often provided at screening health fairs.