Sometimes it's hard to distinguish between bipolar symptoms vs. emotionality. It's something that gets easier with time as you learn how to live with it and discover what's your disorder and what's you.

My experience has been that it's the bipolar when I get wound up and there's no specific trigger, although I have been thrown into mood episodes because of severe stress. My mood episodes also last for days or weeks, while an emotional reaction will usually last only hours to a couple of days. Also with bipolar swings, I notice a gradual change in my mood and energy levels over a period of a few days, while with emotions it's instantaneous.

Hope that's helpful. As to how one knows for sure whether or not they're bipolar, I don't know that there really IS a way to be 100% certain. A diagnosis is nothing more than an educated statement by a health professional that they believe you have a certain condition. For me, the easiest way is to look at my online mood chart and see the huge ups and downs over the course of the past two years. It looks like a wildly irregular EKG tracing.