Im a noob

clinic , hospital , gym , buy ur own scale

No need to.

I was asked that question before.

I proceeded to tell the questioner that no medical professional try to ascertain an individual's body fat PERCENTAGE for ANY indication for general clinical conditions whatsoever (i.e. measuring obesity, or even weight-loss or exercise progress). There ARE scientific methods of ascertaining body fat:

Measuring Obesity: From Calipers to CAT Scans, Ten Ways to Tell Whether a Body Is Fat or Lean

But you will note that even in the more technologically-involved methods, they are mainly RESEARCH methods, not individual CLINICAL methods of body fat measurement (DXA is used clinically, but for bone density [osteoporosis]).