So who is your target?

Was watching Sex and the City, the episode where Carrie is picking the columns and book cover for her first book, and has to figure out who her target audience is. 
I have seen on blog discussions this question.  And up until a hour ago, I always thought I knew mine.

Figured my target audience were Sex and the City fans, gay men and wrestling fans/wrestlers.  Ends up that might not be the whole story.

I asked Mel what he thought (yeah I know, I'm an emotional cutter) and his answer  " 18-30"
Um yeah, that doesn't help me bud.  

Makes me wonder though if that is a good thing or a bad thing, not really having a solid readership.
Am I accessible to a wide range of readers (good thing)   or am I just not what people need, like the self help section of the local book store (bad thing)

Mel believes and this is a direct quote.  "Good thing. That way it never gets stale and you reach a broader range of people and age groups"


How do you know you've reached who you thought you were reaching with your writing?