I asked during a recent appointment. Doctors don’t really test everyone for diabetes; if a normal blood / urine test reveals blood sugar levels that indicate diabetes they can order / direct one or more of three different tests to confirm the results. If your doctor hasn’t brought it up, you are likely okay. IRT your brother, did he get regular check ups & routine testing? Y father was diagnosed with diabetes in his 50s, he rarely went to the doctor but looking back the mid atoms were there for years.
I am far from a doctor, but going on 15 years when my doctors reports to me my physical results (blood pressure, cholesterol, blood sugar etc.) I ask if it is normal.
All that aside, best wishes for your brother.