Take a tissue sample. Isolate a single cell. Look at the chromosomes under a microscope. See if I can find a Y.
Really, that's it for me.
I'm not noding this to be willfully naive or ultra-conservative or flamebait or anything like that, so please just read before you vote. I'm trying to make a sincere point here.
I am a guy. One X, one Y, a penis and two testicles, testosterone, and all the secondary sexual characteristics that derive from the above. Sexually attracted to women. Born, raised, and lived my whole life as a male of the human species. And for as long as I've been alive, save for the past few years, the above has been sufficient to define me as having male gender.
It's only recently that I've been exposed to the notion of sex and gender being two separate things. But the vast majority of the planet still uses them interchangably, and for a very simple and obvious reason.
So here I am, a young adult living on the cusp of the twenty-first century, trying to integrate my understanding of the word "gender" with homosexuals, bisexuals, transsexuals, transvestites, and every other variation on the sex/gender pairing that can possibly exist. Thanks to the Internet, I've known one person from just about every category. I've watched most of the movies. I've considered all the possibilities. I've opened my mind about as far as it can go without snapping off the hinges.
But do you know what I keep coming back to? "Men have penises, women have vaginas." "Men should be attracted to women and women should be attracted to men." Anything else is and should be considered an anomaly. Not because that's the way I was brought up or because that's the position my religion holds, but because that's the way I am.
Sex, gender, orientation, they're all the same to me. Doubtless they always will be. Asking me to define my gender separate from my sex is like asking me to define the clouds separate from the sky.