Not everyone has a gemological microscope to have a closer look at the drill hole of a pearl to see if it says anything about its validity. That is why it's a great idea to own a 10 power loupe so you can magnify pearls in low tech.
Here is a typical artificial pearl under a microscope. Without magnification, it looks pretty good, but zoom in and you can see that the smooth surface is disrupted near the drill hole.
A real pearl has structure to the nacre that causes it to fracture under...