I remember when I used to ask this question.. There's only one answer.
You just know.
I kno, that answer sucks, but its true. If everyone could be dancers, if everyone could be singers, if everyone could be rappers, if everyone could be musicians, if jus anybody that could afford a sequencer could be a killah producer, there'd be nothing special about it.
Not everybody is musical, but jus like most skills, you're bound to have at least a little bit of it in you.. you'd just have to train it. Now, thats no guarantee, but it may be there. Hopefully you have enough time on ur hands to do this.
Get familiar with music. Not jus listening, but if u listen, listen closer. Notice the bassline and the melody. If you can, get an instrument and learn to play. If you plan on being a producer, you probably have a keyboard. Play it, learn some songs, and then you'll pretty much be able to feel which notes complement others.