Do you see a therapist or Psychologist at all? Even if it's a group therapy program for person's that are diagnosed with borderline. You may get some ideas on how to tell the difference between the 2 illnesses.

It may also help to keep a mood chart (if you don't already). From what I've read the moods with borderline tend to change much quicker than a person with bipolar, so by charting your mood and noting triggers it may help. I use this online mood tracker:

http://www.medhelp.org/land/mood-tracker