Test out the Healthy Body Calculator®. The body fat section tells you how fat can be measured. Also look at the BMI section. You should be able to find more details by searching on the net or in your library for this assessment methods.
Unfortunately, if you are younger than 18, it will not do any calculations for you. So to see info about how to test one’s percent body fat, pretend for now that you are older.
Also, have you read my nutrition topics like exercise, sports nutrition and overweight for more info?