Background: Diabetes mellitus is a complex disease with different pathophysiological causes. Research in recent years has described various pheno- and genotypically defined subgroups for both prediabetes and diabetes. Question: For everyday clinical practice, it is essential to recognize the different phenotypes. It is proposed to do this using a simple measurement of endogenous insulin secretion. Materials and methods: The fasting C‑peptide/glucose ratio (CGR) can be used to estimate endogenous insulin secretion. A prerequisite for this is preserved kidney function (GFR > 50 ml/min/1.73 m2). Results: Autoimmune diabetes and its subtypes can be recognized by a low C‑peptide/glucose ratio, representing a lack of endogenous insulin secretion. A low CGR also describes severe insulin-deficient diabetes (SIDD). In contrast, severe insulin-resistant diabetes (SIRD) is characterized by a high CGR, which is equivalent to insulin hypersecretion. Conclusion: The C‑peptide/glucose ratio allows a simple assessment of different pathophysiological forms of diabetes. Importantly, it can also provide additional information on the need for insulin therapy.