You don't have to be below 10% to have all visible.. every body is different. I'm a personal trainer, 12% body fat and everything is visible.
Also depends on how much bulk your abs have (muscle mass) which is why you need to train them, water levels, etc..
I've seen people with 15% body fat and their abs are completely visible.
Back on topic..
- Calipers or body stat machine at the gym will give you body fat.
You could also try this..
http://www.bmi-calculator.net/body-fat-calculator/
Not very accurate though.