16. Real breasts have more sag. The tend to be less likely to face forward than unnatural ones, the nipple tend to be lower down because it's moved during surgery, and of course perfectly shaped teardrop breasts are usually fake.
It's generally the rise at the top that gives away most boob jobs. Those of us who are natural have this force that prevents it. It's called Gravity. That, and you can tell if they've had sculpting done.