I think I may be bipolar. How can I know for sure?

See a psychiatrist. Not always easy 2 make diagnosis bring record of mood swings & close friend or relative who has observed u over time.
There are many ways. Best to discuss with your doctor. Depends on your age. There are many good web sites, but the nimh & nmha probably the best to check out. A recognized screening tool is the mood disorder questionnaire. You can fill it out online if you're an adult: http://www.Nmha.Org/bipolar/questionnaire.Cfm. Print it out and take it to your doctor.