A simple test of blood sugar measured after an overnight fast, this is a measure particularly good at picking up diabetes.  Levels above 5.5 mmol/L (99 mg/dl) indicate a higher chance of diabetes. There are however many, many individuals with normal fasting glucose who have significant insulin resistance and pre-diabetes.
The Oral Glucose Tolerance Test is commonly prescribed to detect diabetes, yet again it will rarely be able to detect insulin resistance or hyperinsulinemia. Many patients with insulin resistance will have completely normal glucose tolerance tests and passing this test does not mean your metabolism is healthy!
In PCOS, a condition where high levels of insulin cause the ovarian theca cells to release high levels of testosterone and other androgenic hormones, it is helpful to know the degree to which hyperinsulinemia is present. Often, women with PCOS have completely normal blood sugar levels, and can often have low blood sugar levels as well. Yet, their metabolisms may be significantly impaired.