Heh... I've been an RN now for almost 2 years and I still really have no clue what my "passion" is and am somewhat afraid to take the plunge and find out if it really is psyche nursing. =) When our hospital used to have a psyche unit, I used to love to get pulled there (I was just a tech at the time). Still, for some reason, I'm afraid to take the plunge and try to get a job at the state hospital. A lot of it, I think, is simply because I've been working at this hospital for six years, in med/surge.
Anyway... I guess the only thing that you can do is try something, and then if you really don't like it.. don't be like me and be afraid to try something else.
In RN school, you'll do rotations on different types of units (Psyche, OB, Peds, Nursing Home, etc..), and that should give you some CLUE.
I even liked my psyche rotation, and I'm STILL afraid. Isn't that weird?