There are a couple of ways to find your body fat %.
There is a hand held machine that you can find at sports stores or online. You plug in your height, weight, age, and sex and it tells you your BF%.
They also sell body fat calipers. You measure certain parts of your body and plug it in to a formula to find your BF%. You can find videos on youtube and the formula on google.
The water displacement test is done in a more clinical setting. You sit in a chair and they dunk you under water. It is the most accurate but the most expensive.
Your best bet would be to find the hand held machine.