In life, people wait for some sort of passion to enter their souls. Actually, they're waiting for the impetus to draw out the passion instilled in their souls from the very beginning. I believe (and yes, this is a rather mainstream belief) that every human being on Earth enters the universe bearing an intrinsic and valuable gift--one that will serve for the betterment of mankind. As a person develops and matures, this gift may surface, and compel the person to follow the path the gift intends for. Some consider this a vocation, a calling. A calling to a certain way of life may be evoked by talents discovered early in life, provocative events occurring in one's life, or even just an unexplainable burning desire to pursue something.

But how do you know exactly what your calling is? Doesn't it seem like you've had a myriad number of these "epiphanies" already? I definitely feel that way. People ask me what I want to do with my life and what I want to study in college, and there are different answers continually coming out of me like clockwork. It's to the point where some may think I'm a compulsive liar or a schizophrenic...seriously. But over the past year, I've finally come to the point of realizing what I need to be doing. Actually, it's what I truly want to be doing. I know it is my calling because I would be happy doing it every day for the rest of my life, salary or not; this calling is what brings out the best in me and empowers me to try and change our world.
I think the best way to know if you've found your calling is to trust your intuition. I.e., it will just feel right, like a lot of things in life. Sometimes there is no logical reasoning for our decisions; we just "go with our gut" and roll with it.
And, still, sometimes, you discover much by simply taking a risk. Why hold back if the best thing for you is out there waiting? Those who play the card of fate are those who will find the most success and enjoyment in life.  So don't just jump; dive.