Does a CGM tell me if I have insulin resistance?

Not directly. The CGM measures glucose, not insulin. A man can have significant insulin resistance, his pancreas producing two to three times the normal insulin to keep glucose in range, and a perfectly clean glucose trace because the compensation is working. The CGM sees the output; it cannot see the cost. This is one of the most important clinical limitations of CGM-only metabolic monitoring.

The correct test for insulin resistance is fasting insulin, or more precisely, HOMA-IR (calculated from fasting insulin and fasting glucose). A fasting insulin above 10 mIU/mL in a fasting non-diabetic man with a clean CGM trace is a finding that changes clinical assessment: it identifies compensated insulin resistance before glucose becomes abnormal, at the stage where dietary and lifestyle intervention has the greatest effect. The HOMA-IR calculator is free; the fasting insulin test costs approximately $25 at any direct-to-consumer lab. (Matthews et al., Diabetologia, 1985)

Cardiologist's calibrated position, Solid (1) for fasting insulin as the correct test. The CGM gap on insulin is a design limitation, not a flaw.

What to do: If you have been using a CGM for more than 30 days and have not measured fasting insulin, add it to your next blood draw. The combination of CGM trace and fasting insulin gives a far more complete metabolic picture than either alone.

For the full picture, read The CGM on Your Arm Doesn't Know Your Arteries.