A good way to tell the quality of a pearl is by it's particular shape, and the luster it has. Authentic pearls are "nature made" and are not perfectly round in shape.

However, the more round it is, the more rare and valuable the pearl is considered.

The authentic pearl will have a natural luster that reflects not only the light, but will reflect the image of objects near its surface. Even the highest quality of authentic pearl will have at least one small surface irregularity, perhaps more. But, "faux" pearls will be exact likness of each other on a strand, without the slight "blemish" or irregularity or difference between them of true, nature-made pearls.