To be honest, I think most writers know early on whether or not they have the talent to do it, just like with acting or being a comedian. Having a desire to do something, even a strong one, doesn't necessarily mean it's something you'd be good at.
For example, no matter how much someone wants to be a stand-up comic, if they aren't naturally funny, most likely, they won't succeed at being a stand-up comic. Sure, you can work hard at it, study other great comics, etc; but most of the time, it really comes down to whether or not you just have the talent to do so.
But to be clear, this can totally depend on what type of writing you mean. Some people may not be good at fiction writing, but may be excellent with essays or biogrophies. Maybe your particular writing style is good for journalism. This is something that may be the case with you: maybe you are good writer, but just haven't find your niche.