Vale, in terms of seeing whether this is a "real fit", I'd agree with Jessica that one of the best ways is to start with therapy. A good therapist will help you open the doors and understand yourself in ways you may not have ever imagined possible. Don't be scared to change therapists after a few sessions if you feel that the vibe is not right. And do try find a therapist with good transgender experience.
The only other way to see if it's a real fit is to live it, but you should really be prepared mentally for this and build up to it. Not only should you be ready mentally, but I'd advocate creating a detailed plan - I did this - where I gradually turned on the girl and turned off the boy. If you're interested, here's how it worked for me:
By January this year I had got to a mental state where I was "90%" sure I wanted to become a girl and was ready to start the process. It took a month to find the right endocrinologist, and he asked me for a psychological report and recommended a specialist psychologist. We had around 6 sessions, once a week, by which time it was clear to her that I had gender dysphoria and was ready to transition. (Careful - 6 sessions was just the cherry on top for me - 20 years earlier I had done 18 months of therapy to find myself.) In January I also started laser beard removal. In April I began HRT and also decided to begin living as a woman. Again, this is unusual - many people spend a year or more on HRT before taking this step. For me, this was my dry run to verify that it was a real fit. For the first month I had frequent doubts - "Are you sure you want to do this?", but as I got better at being a woman those doubts left me. (It's important to note that becoming a woman requires effort - we have to unlearn masculine behaviours and learn feminine ones - for many of us these don't come naturally.) By month 2 I was totally comfortable and astonished at how easy it had been for me to reach this point. I began a transition plan with HR at work, came out to my family, and by the end of July my plan is for the boy to disappear at work and only The Girl will remain. I spent hours creating my plans, and developing backup strategies for family and work, just in case. Now I have no doubts at all - I know that this is the right decision, and my daily life validates and vindicates it. Despite the ongoing stresses of transition, I am dealing with it with a sizeable dose of humour and am enjoying the journey. My biggest challenge? My voice. But after 6 weeks of voice training with a singing teacher, and some darn hard work listening to women and training myself, it's coming right and I am thrilled.
In other words, good planning, hard work, and a dose of luck will help you get to the place you need to be.
Hugs
Julia