This is an excellent question. I feel like I haven't found it and I'm 68! I better hurry up. It's there; I know it. Right now, though I'm disappointed in my "talents".
When I was teaching Adult Basic Education for the Deaf at one point, I had the experience like Billybuc and Bronterae describe. One or two times in that evening class where I taught without a word spoken, (so to speak) I felt "in the zone" as Mihaly Csikszentmihalyi describes in his book, "Flow," but it was fleeting.
There recently was a piece on TV the other day that featured a person who had formerly been in jail who was extremely positive about what he had to give to the world. The commentator seemed to doubt him, but he was adamant that he knows he was called to do something important. Actually, I feel the same, but I can't put my finger on it. I'm still searching. As they say, "I hope my ship comes in before the dock rots." If you're searching yourself, you might find the Strong Campbell Interest Tests helpful for a general ball park. Also, the Myers Briggs has an in depth career assessment that goes along with its personality test, but again, that only puts you in the ballpark. It will be interesting to see what others have to say here.