There are several ways to identify authentic pearls, the first and safest being to purchase them from a reputable jeweler.
High-quality authentic pearls are extremely reflective under the light, and exhibit variations in iridescence and color throughout the surface. Pearls that are perfectly uniform in color and overtones, or demonstrate chipping on its top layer, are imitation.
There are gemological experts that are certified in pearl authentication.
Lastly, you can also determine if a pearl is authentic or not by rubbing two pearls against each other. Both pearls should feel as if they have a grainy texture while you do this.