There are several ways, you can go to a doctor and ask to have your bodyfat tested and he/she would do it, and depending on the doctor the ways they might do it would vary.
There is a machine(name escapes me at the moment), but it's like a scale, and you stand on it, and it will print out a little reciept basically of your total
body fat, your body fat %, your total body water, etc...however, these are not very accurate. I personally reccomend going and getting yourself some calipers, those are the most accurate and their cheap and easy to use! Their like little pincher things basically

...Type "Body
Fat Calipers" into Google and see what ya get.
As far as keeping bodyfat to a minimum...Two things; Diet...and Cardio
Let me repeat this - Diet...and CardioDiet...and Cardio Diet...and Cardio Diet...and Cardio Diet...and Cardio Diet...and Cardio Diet...and Cardio Diet...and Cardio Diet...and Cardio Diet...and Cardio Diet...and Cardio