How do you know for sure if you are diabetic? what blood test do you need?

Various methods: There are various criteria including fasting glucose: random glucose, post meal glucose, hba1c that looks at prior 3 mos of glucose. The fasting glucose is the oldest and most established diagnostic tool. Fasting over 104 is abnormal. Post meal should be 130 or less. Glucose of 200 or above at any time is probably diabetes. See diabetes.Org.

Fasting Blood sugar: Usually the diagnosis is made by testing for blood glucose in fasting state. Glucose tolerance test may be needed in some and hemoglobin a1c may also be used. See this site for more info http://www.Mayoclinic.Com/health/diabetes/ds01121/dsection=tests-and-diagnosis.