Answer:
Quick and dirty: Are you a boy or a girl? Do your sex organs match the sex you are or are they opposite. If they don't match, then you are a transsexual (or transgendered) if they do match, then you are not.
A lot of people in the TG (transgendered spectrum) end up asking themselves this. In general if you have to ask, then you are not. One of the very basic things people own about themselves is their sex. Ask someone in front of you what their sex is, and you are gonna get scary looks. Most are of the type "DUUHHH, are you stupid? It's clear I'm ......" Because your sex is so intimately intertwined with your identity, since 95%+ of the people look the sex they identify themselves with (and have the parts), in most cases people have 0 question about their sex. If you feel you might be a (opposite sex) and don't believe with all your heart you are one or the other, you are not a transsexual.