A glucose tolerance test will show how your body reacts to a set amount of glucose/carbs. They will test you fasting, 1 hour, 2 hour and 3 hour to see how you metabolize. In that test, they are doing blood sugars. That's why I asked if you checked yours because, in essence, you can perform this yourself. I don't know that they are measuring anything but blood sugar during a 3 hour GTT.
You can have a c-peptide test (blood test) which measures your insulin production by your pancreas. Those with Type 1 diabetes show no to very minimal insulin production. Those with insulin resistance usually have normal to high insulin production with higher blood sugars. So, perhaps it's your insulin production that should be measured.
I hope I'm not misunderstanding the types of tests to which you are referring?