Step 1: Real Boobs have a natural sag to them, they are fuller on the bottom then the top. If a breast keeps its perky roundness, there is an implant holding it up.
Step 2: Fake boobs or breast implants are very identical and look nearly perfect. Real boobs are not always perfect.
Step 3: If cleavage is rounded, and looks inflated, it's fake.
Step 4: If a skinny girl has DD breasts, they are fake, it does not happen naturally.