How do you measure body fat content?

i am getting ready to start the p90x program, i want to know how to measure body fat content. it says there are a few ways (US NAVY, 3-site, 7-site, and digital scale) could someone please explain these on at least on in detail?

Public Comments

1. Just go to BMI.com and it will show you the easiest way. You divide your weight by your height.
2. Body frame sizes are categorized as small frame, medium frame, and large frame. The reason for this is that bone structures vary in size and density from person to person; and from male to female...
3. I have posted my blog obesity-treatment. you can find you answer there. http://obesity-treatment-online.blogspot.com/