I think the blood fasting test that SnowyLynne is talking about is the GTT (glucose tolerance test). It is a more accurate test for diagnosis than simple fast (8 hours then blood draw) or the A1c. The A1c is not used as a diagnosis tool because it is not standardized, the GTT is. With the GTT, you fast all night then go to the lab. They give you a cup of sugary liquid to drink and then they draw your blood at different time intervals to see how you are reacting to the glucose in the drink. There are a couple of different timed protocols, but they all work in the same basic way.
This allows you doctor to see how your body actually reacts to sugar. It is my understanding that folks who are prone to diabetes show abnormalities in how they process sugar a lot sooner than they show up with high fasting numbers.