As others have said, the c-peptide test can give you a rough indication how how much insulin is being produced by your pancreas. I have had the test a number of times. The first time I took it, my result was 1.6 ng/ml at a blood sugar of 131 mg/dl. Knowing the blood sugar at the time of the test is very important. In my case, the test pretty my said that I was insulin deficient. But am I insulin deficient because my pancreas was not making enough insulin? I don't know. I clearly was unable to produce enough basal insulin to get my fasting numbers down. But does that mean my pancreas is pooped out., it may mean that my body has messed up signaling. There are after all many defects involved with diabetes, including defects that mess with your bodies set point for normal fasting blood sugars.
Ralph DeFronzo, a well respected diabetes researcher, suggested in his 2009 Banting Lecture that by the time you are diagnosed and have a postprandial blood sugar over 200 mg/dl at 2hrs you will have lost 80% of your beta cell function. So, let say it again to be clear. DeFronzo argues that by the time a person is diagnosed with type 2, they will have lost 80% of their beta cell function. So if you are here, you likely have lost significant beta cell function.
While you might worry and focus on your poor pooped out pancreas, it is very hard to know exactly how healthy it is. But you can know exactly how "effective" it is by closely monitoring your blood sugar. If you only have 1% of your pancreas left, but you still do the diet, exercise and medication properly and get an A1c of 5.5%, then your pancreas is still "effective." And that is really what matters. We work with what we got.
ps. And the c-peptide test can be used to see if you make "any" insulin, but almost every type 1 diabetic still makes a little insulin. A recent Joslin study of type 1s with diabetes for 50 years found that the majority of them still had a measurable c-peptide.