C-peptide
The C-peptide test is a tool your doctor uses to test whether you have type 1 diabetes, when the immune system attacks and destroys cells in the pancreas, or type 2, when your body doesn't use insulin as well it should. It shows how well your body makes insulin, which moves sugar (or “glucose”) from your blood into your cells.
The test can help your doctor decide whether you need to take insulin to control your condition.