If your condition dramatically improves on bipolar meds, that's a good suggestion that you're bipolar...

If your symptoms aren't clear-cut, it can be all but impossible to tell without a psychiatrist, and sometimes even they have to make nitpicky distinctions to reach a diagnosis. In my case, it was the persistent insomnia for the first two or three hours of the night that told my doctor I was bipolar. Lo and behold, drugs like Lamictal and lithium and Abilify worked much better than antidepressants alone to get me functioning (more or less) again.

Not that anyone expects you to start popping lots of different pills to find out. Just that the success of my meds corroborated the doc's diagnosis.