janet,
There is no simple way to diagnose for bipolar disorder, it may take one time or several times before a psychiastrist or a trained professional comes to a diagnosis.
You've mentioned that her moods get really bad around here period and it does for millions of people out there. It could be Premenstrual Dysphoric Disorder (below are links that explain a little bit about PMDD):
http://www.drdonnica.com/display.asp?article=1086
http://www.emedicine.com/med/topic3357.htm
She could have any of those things, depression, bipolar etc, but there is no sure way of knowing without a psychiastrist or a trained professional handling it.
What symptoms is she having that is making you suspect that she might be bipolar?
In the mean hand, I wouldn't mention anything just yet, I would do some research on PMDD, bipolar and depression before approaching her about it. Then suggest that you go see a psychiastrist or a trained professional together if this might make it easier for her.
Best on your search!