Brief Answer:
C peptide

Detailed Answer:
I see you are medications for type 2 diabetes.

A random C peptide blood test is not very useful. The correct way to interpret a C peptide result is to perform a Glucagon stimulated C peptide. This is often reserved for research settings and rarely performed in clinical practice.

C peptide is a surrogate marker for the body's insulin producing capacity and one tends to deduce low insulin reserves when a low reading is observed. However, such a random sample has many limitations and can be misleading.

Further, this type of C peptide result alone does not make you a type 1. The hallmark of type 1 diabetes is the presence of antibodies in the blood such as anti GAD65, anti Islet cell 512 and Insulin auto antibodies. These destroy the insulin producing beta cells of the pancreas. Typically this occurs rapidly and C peptide is virtually undetectable. But it can occur slowly too in which case the C peptide declines gradually.
In type 2 diabetes too, over the course of several years, C peptide levels may tend to go down and many individuals with type 2 diabetes eventually require insulin for control.
So on the basis of this one C peptide result alone, one cannot distinguish between type 1 and type 2 diabetes. Clinicians take into account a broader history and presentation, in addition to above antibody tests before making the final call.

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Brief Answer:
Follow up

Detailed Answer:
As alluded to above, it is not a good idea to rely on blood C peptide levels to make this determination as to whether a given individual will benefit from Insulin, although it is possible some practitioners may deploy this method.
I find it far simpler and cost effective to try medications in the 'secretagogue' category such as glimepiride and repaglinide (to name two representative drugs) and assess the response, provided other oral agents have not succeeded in controlling the glucoses

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