Question: What tests (blood tests, genetic tests, brain scans, etc.) are used to diagnose bipolar disorder?
Answer: So, which tests are used is actually an easy question to answer, because the answer is none.
The diagnosis of bipolar disorder is a clinical one, and made in consultation with your mental health professional.
So there are no blood tests, there are no genetic tests, there are no brain scans, EEGs -- things like that that would be useful. Certainly those tests might be ordered as part of a general evaluation and workup to make sure that there are no other medical conditions that could be causing the symptoms of bipolar disorder, but ultimately, bipolar disorder is a clinical one made in discussion with your physician.