A recent study found that there are a number of factors that increase the chance of having bipolar disorder.
Having at least four previous depressive episodes, suicidal acts, cyclothymic temperament, family history of bipolar disorder, substance abuse, younger age at onset and male gender all significantly and independently differentiated bipolar from unipolar disorders in the study of 2146 patients who initially presented with a first episode of major depression.
After an average of 13 years, 642 (29.9%) patients were diagnosed with bipolar disorder and 1504 (70.1%) were diagnosed with major depressive disorder.
Further statistical analyses showed that differentiation of future diagnoses of bipolar from unipolar disorder was maximal when between two and four risk factors were present per person.
I, myself, have about three. A family history of bipolar disorder, a younger age of onset and suicidal thoughts. The list specifies “suicidal acts” but I think suicidal thoughts could possibly count as well.