Dreamer vs. Writer vs. Author
I'll confess, it always bugs me when somebody comes up to me at a signing or a speaking venue and whips out that hoary line: "I've got this story in my head. I know it'd be a really great book, and if I get the time someday, I'm going to write it." The subtext is generally something to the effect of: "I'll be on Oprah with my first try, have People pounding at my door, and won't give a working hack like yourself the time of day."
These folks are the dreamers, of the same ilk as, well, me, when I fantasize about playing backup to Bruce Springsteen (I was born and raised a Jersey girl; this dream comes with the territory) in spite of the fact that I have no musical talent whatsoever. The dream is really vivid (including tanned and beefy lifeguard groupies eager to offer up a standing O!), but I haven't put the time in, don't intend to put the time in, and lack the genius for going beyond the printed note.
Much better than the dreamers are the writers, who not only have the dream but work their butts off to achieve it. They know what it is to lose themselves for hours in a story, for weeks or months or years on end. They read the craft books, subscribe to Writer's Digest, attend the classes when they can. Some of them will go on to become authors, but others, sadly, won't. Maybe they can't bring themselves to put the dream first. Maybe they can't bring themselves to risk rejection, or they let a few rejections or some creative writing teacher or contest judge's criticism crush them. Maybe they can't take the snidely "helpful" comments of their spouses or their friends or relatives. Not everybody has the strength and constitution to able to continually pick herself back up after smacking her head against the brick wall. And, sadly, some of those who work hard and accept the risk won't become authors because they simply lack the spark, or enough true spark, to coax into a flame.
And then there are the authors. These folks are the ones who work long and hard, who educate themselves the craft and the market and the business, who persevere in spite of all discouragement, and who have enough native talent to pull together the whole package. They gut out the long, tough process and hone their skills until they finally, finally (oh, blessed day!) find a publisher eager to pay them (sorry, but if you're paying the publisher, it is definitely not the same thing) for the privilege of turning the fruit of their dreams and hard work into a book.
So how do you know if you'll be a member of the last group? You don't - you can't = for sure, though there's a stubborn little corner of yourself that feels it, that believes it, that absolutely will not take no (even hell, no!) for an answer. If you've got that much on your side, at least you stand a chance.
And a chance is all the guarantee you're going to get.