I suspect my brother is taking insulin though he's non diabetic. how can we determine that he's taking insulin? do we have any diagnosis for it?

Mis-using insulin: When non-diabetic people use insulin, it's called Factitious Hypoglycemia. Blood tests that can be done in the setting of hypoglycemia (low blood sugar) that can confirm this are: plasma insulin, C-peptide, proinsulin. This will differentiate between mis-use of insulin & an insulin-secreting tumor called insulinoma. Encourage your brother to see a doc. If misusing meds, consider psychiatry.