Many human beings have an easier time discerning what they're not than figuring out what they are.  As you relate to this mindset, you may ask when it all began? 

Back during school days, you may recall going through a process of elimination.  You may have used this technique on multiple choice tests.  You may have used this technique to figure out what kind of job, trade or apprenticeship made the most sense at the time.

What about the comparisons you unconsciously make? Many human beings look in the mirror and compare themselves to people around them.  You may judge yourself based on invisible standards. If you aren't built like an athlete, then you may tell yourself you aren't meant for sports.  if you don't achieve high grades in math, then you may decide you aren't destined for banking or accounting.  Does this lead you closer to who you are?

There are always some people who are convinced they know who they are from a very young age. These human beings demonstrate confidence and seem self-directed.  They may score well in particular subjects and obtain the diplomas necessary to follow a chosen path. Yet, does one kind of fulfillment mean they know all there is to know about them?

The short answer is only you can answer the question about yourself. It isn't based on what other people believe, approve or disapprove.  it isn't based on what you could be doing if you weren't doing what you are.  Knowing yourself is a feeling, a state of awareness that is growing.  This isn't complacency. It isn't based on an assumption that you've already done all you need to do.  There's so much more than you think you know.