If someone is experiencing a depressive episode, the assumption is usually that it's a unipolar depression (as opposed to bipolar disorder) unless there are really strong indicators of bipolar disorder (for example, the patient has spoken with her family doctor about her mother who has bipolar disorder; or there have been very clear symptoms of hypomania or mania in the past).
The reason bipolar depression is often mistaken for unipolar depression is that people tend to not go to the doctor or seek help otherwise when they're feeling good - ie. when they're hypomanic; so quite often, people go misdiagnosed as having unipolar depression. In one study done a few years ago, the average person with bipolar disorder went nine years with an incorrect diagnosis!
If you believe you have bipolar disorder, it's best to see your doctor armed with information. Start by going to www.dbtforbipolar.com and completing the Mood Disorder Questionnaire, a screening tool for bipolar disorder. Print out your results and bring it to your health care professional.
It can also be very helpful to have someone at the appointment with you to provide collateral information - in other words, to provide another perspective on the changes that happen in you when you're not feeling emotionally well. Of course the doctor will want to know how you experience your symptoms, but having someone else who can provide observations will also help to make an accurate diagnosis.
Finally, just be truthful. Whatever label gets stuck on your problems, they're the same problems you had without the label - now you just have a name to call them by!