Blood tests. Although they have some things in common. The first step is a physical exam and some simple blood tests for thyroid status. So go to your doctor and let them evaluate. You also can have both, but clearing up thyroid and other physical problems will help either way.
Evaluation. Thyroid problems are diagnosed with a history, exam, and lab tests, possibly and ultrasound. Bipolar is diagnsed with a hisptory, exam, and ruling out other causes (like thyroid).