What methods are used to diagnose hypoglycemia?

Blood sugars. Hypoglycemia means low blood sugar so you have to have blood sugar levels to make the diagnosis. Sometimes a single determination can suffice but it might require a glucose tolerance test to detect reactive hypoglycemia.
Hypoglycemia. We diagnose hypoglycemia if a patient has (1) appropriate symptoms of hypoglycemia, (2) documented low blood sugar, either by fingerstick or lab test, and (3) appropriate positive response (resolution of symptoms) with administration of glucose/sugar.
Depends. On whether fasting or after eating. Talk to your doctor to determine if you actually have "hypoglycemia".