Yes by HbA1c we can diagnosis diabetes but this test needs to be done at a NGSP certified lab . Normal HbA1c is less than 5.7 % . Between 5.7 to 6.4% is pre diabetes and 6.5 % or above is diabetes . Other tests are blood sugar fasting ( 126 or more ) 2 hr PP ( 200 or more ) , random blood sugar of 200 or more with symptoms . And OGTT after consuming 75 grams of glucose if its 200 or more after 2 hours .