Before a diagnosis is made your doctor will need a blood sugar reading of 2.4mmol or under, this is a hypoglycemic blood sugar level. The value may vary slightly depending on where you are. You should also experience symptoms which are relieved by consuming glucose.
Typically your doctor will call for a GTT first off. If this doesn't show anything, you'll need a 72 hour fasting test for fasting hypoglycemia. If this is negative, you may be diagnosed with reactive hypoglycemia depending on your doctor and what evidence he has for the diagnosis at that point. Or further investigation may be done.
Describe all your symptoms to your doctor, what times of the day that they occur etc.
It's very important that both the type of hypoglycemia, and the cause are found.