For differentiating between Insulinoma and Sulfonylurea related hypoglycemia, the test which is useful is ?

Surreptitious use of insulin or hypoglycemic agents like sulfonylureas may be difficult to distinguish from insulinomas. The combination of proinsulin levels (normal in exogenous insulin/hypoglycemic agent users), C-peptide levels (low in exogenous insulin users), antibodies to insulin (positive in exogenous insulin users), and measurement of sulfonylurea levels in serum or plasma will allow the correct diagnosis to be made. Reference : page 569 Harrison's Principles of Internal Medicine 19th edition