I don't 100% know, but I'm at about 80%.
I look at pictures of men and think "I want to be that". I see how men are treated in media, in real life, and I think "I want to be treated like that". I see the way men stand and walk and speak and think "I want to be like that". (Well, more "I do that already, I wish people wouldn't think it's funny!") I see male facial structure, musculature, and I think "I want to look like that". I don't experience acute body dysphoria, and I don't hate my birth junk, and sometimes I quite enjoy putting it to use with the right partner, but my chest is "bizarrely" devoid of sensitivity, and being treated like a woman in bed has always been kind of a turnoff. I think I dissociate a bit when I look at pictures of women, and something about looking at porn with vaginal penetration or seeing images of vaginas has always made me really, really uncomfortable.
Who am I at the end of the day? Well, I'm me. Where I go from here has nothing to do with being "100% sure" of anything; life is a gamble. Realizing that transition is a series of small decisions helped. If you think you're a guy, start with mental exercises. Body language. Try a binder on and see if you like what you see in the mirror. Start doing upper body exercises or practice lowering your voice. It's not all or nothing, RIGHT NOW. Transition is one day at a time.