Is this a trick question? - Let it go outside, and if after a few months you find a pile of small furry kittens in your airing cupboard - then you've got a female. If it just grins a lot, then you've got a male. - Seriously though, you could always roll it over on the floor and check it's genitalia. None = female, Anything vaguely resembling a caterpillar stuck between two furry cushions = Male.
personally i think if a cat is male it will have a more masculine build ( broader shoulders and face), and also a better posture and deeper meow. if a cat is female it will have a feminine build and sweeter meow?? lol
if it keeps walking round squirting a foul smelling pee up the furniture its a male, if it keeps dropping little fluffy things round the back of the afore mentioned furniture then its a female. to stop any of the above get it down the vets snip snip