While fake boobs have really perfect round shapes, natural breasts follow a more consistent sliding curve line. From top to bottom.
No natural breasts in this world are identical or symmetrical. The day your friend is wearing a low-cut top, watch how her perfectly round boobs move as she moves, especially when she walks and bends down. Because natural breasts are soft, they jiggle wiggle when you walk.
Notice her boobs when she bends down, as real breasts gravitate. If her breasts look like 2 cups superglued to the saucers, then you know her pair of Louis Vuitton is from China. As breasts are mostly made of soft adipose tissue, How to know fake boobs will change when you gain weight, are pregnant, breastfeed or on estrogen pills.
There is plastic on that rack if she is stick thin with heavy melons hanging.
Our bosoms should be at about our underarm height. This a classic tell-tale sign.