You have to meassure it. 8 to 15 is healthy eventhough the number doesnt say all about your health. A too high number though clearly is not healthy.
A healthy percent is down 20% ... You can see a nutritionist if u want!
i found this body fat % calculator online
not the most accurate way, though
1-5% is what's considered unhealthy having something to do with your body not having enough fat to support itself.