Time for me to kick in my two cents. I would have to say that I didn't know where my interest lied until I actually got involved in the life. Real-time, face to face. I think that there's nothing that can really equal the value of experience. That when people learn and associate.
As far as when I knew I was a Dom, I started in the life as a sub. I was a bitchy, arrogant little bastard. My Papa taught me how to grow up and become a man. I'll have to say that we wore out a lot of floggers to get me to that point. And then, it was like something clicked in my brain that I was meant to do the same for others. That it was about spiritual growth.
After he died, I was charged with his "mission." Now, that's not to say that every sub needs to be turned into a Dom. As a matter of fact, very few people go that direction. But what the mission is about is teaching people to become masters of their own spirit, whether it be in service as a sub or in Service as a Dom. BDSM acronym = Better Discipline for Spiritual Mastery.