I had such a hard time with this question myself, because I lived happily as a man for years and years. (Looking back, there were inklings, sure. But I'll never know how much of that is just that hindsight is 20/20, you know?) Then I literally sat bolt upright one day when I was 32 and had a revelation as everything coalesced - I was a woman and wanted to transition.
What worked for me, and I don't know if it will help you, was to stop asking "Am I a [true] transsexual?", because there are about a million ways to define the answer to that question. And by most of those ways, I was told I was NOT. Instead, I kept asking "is this next step of transition [therapy, hormones, laser, etc.] something I think will make me happy?" As long as I felt it would, I'd take that step, and then see how I felt. I also decided it was OK to stop any time I decided I'd had enough, or not to take a particular step if it didn't feel right to me. In the end, I fully transitioned within 11 months and had GRS last year, so I guess now I can safely say I'm a real transsexual.

But the thing is, you don't have to answer that huge question all at once either. Ask yourself what you think you'd most like to try, to see if it makes you happy... and experiment.