I did get a doctor to do a c-peptide test a few months ago. He had no idea what it was and I had to explain it to him. Turns out I am producing insulin. Don't know how much - I wasn't privy to that information. I have tried fasting on just water (I reduced fluids as well at one stage, because I have a problem with excess fluid making me swell up, and ended up in hospital). Oddly enough, my BG stayed high for most of the day when I did that, before dropping like a stone in the early evening. Then had to have some food - not much but possibly not the right thing (oatcakes) to put brakes on the drop. Then up it would go again. 2 oatcakes, with no insulin gives a rise of 21/2 - 3, so something like 2.8 to 4.8 or 5.
Edit: Sorry - the postman came (No 2 son) and I had to stop. I was about to say that this is why I am trying to do 18 hour fasts, but I still need fluids.