any girl has the physical capability to squirt. many of them dont have the mental ability to be able to relax enough to let it happen. from what i understand from past gf's the sensation just before squirting is much the same has having to pee. so not wanting to make that particular mess in the middle of the fun the suppress the urge and hence do not squirt.
another small quirk in the female form is the fact that in order for them to cum they must feel an emotional/physical attraction to the person, or at least be able to fake that sensation in their brain if they like to sleep around. which is why it was said earlier "im not even sure all women can cum
usually the quickest way to bring a woman to a screaming squirting orgasm is with a combination of clitoral stimulation either with the tongue or fingers and stimulation of the g spot (which for those unfamiliar with the female anatomy is just inside the opening on the pubic wall and generally has a slightly different texture than the rest of the skin) but this can vary greatly from female to female so the best way to figure it out is to experiment as often as possible.