There are quite a few ways to figure out if a pearl is real, or not. You can weigh it, use a magnifying glass to check the surface, or inspect the shape. Fake pearls tend to weigh less along with having smoother surfaces. I'd also be suspicious of a perfectly rounded pearl as nature is rarely perfect. You can find more information here: http://www.ehow.com/how_2053384_tell-real-pearls-from-fake.html
First you can run the pearl over the biting edge of your teeth, if it is completely smooth then they are fake. Another test is to place the pearl in a glass of soda and see if it disolves, if it does then its real.