This article is about real pearls, both natural and cultured. First, the term 'pearl' should be applied only to natural pearls, formed in a mollusk when some event in nature induces the oyster or mussel to form a pearl. These pearls are quite rare and are the pearls of antiquity before pearl farmers learned to culture pearls. Freshwater pearls fished from the rivers in the USA are natural pearls, but now-a-days, most freshwater pearls are also cultured in freshwater pearl farms.
In any event, naturally forming or cultured, such pearls are formed in the oyster or mussel and are not fakes. The real difference in the totally natural pearl and the cultured pearls is what makes the pearl start to grow and the thickness of the nacre. Nacre is the pearly layers that make a pearl so lovely. The natural pearl will have very thick nacre, generally forming in concentric rings around some starting source or irritation to the mollusk. A large natural pearl is very, very rare. Cultured pearls generally use a 'core' of mother-of-pearl, the shell of the mollusk.
The bead core goes into the mollusk and nacre is formed around the core. The core determines the overall shape of the pearl. How long the oyster is allowed to let the pearl grow determines the thickness of the nacre. Thicker nacre is better. Best of all is thick nacre and a smooth, almost blemish free surface. Pearls grown inside the mussel or oyster do have a gritty feel to the tooth.
Fake pearls are made by coating a core of some sort (mother-of-pearl, plastic, etc.) with a plastic, a kind of paint containing ground fish scales or other stuff that look pearly to the eye. Generally, the fakes do not have a gritty feel to the tooth. Most may be identified with a little practice and a 10x jewelers loupe or microscope. The way to do it is to compare real pearls with some known fakes. The fakes almost always show a grittier look to the surface and not the more smoothly formed natural nacre.
The look is grittier but the feel to a tooth is smoother. After some practice comparisons, most fakes are fairly easily identified. Still, some are hard to detect without very thorough testing sometimes using solvents and acids which attack the real pearl nacre but not the plastic fakes and visa versa.
Other than that, a look at the drilled holes in the beads will help. Since the pearl is formed naturally without a hole, the hole must be drilled. The surface of the pearl at the drill hole will not 'sink in' or 'rise up' as is seen in many fakes that are formed and coated with the holes already made to the core. In the real pearl, the nacre will be smooth with the drill hole and might even be chipped a bit. Some folks recommend having a dentist do an x-ray to see what is inside.
Still, unless a person is very, very familiar with reading the images the results are very iffy. A core will look like a core whether in a fake or a cultured pearl. A core will not be present to any large extent in a pearl formed freely in the ocean but those pearls just are not around or to be had except in very small sizes.
Using a good loupe (hand held magnifier) of 10 power is likely the best home test. Of course, try the teeth first! Then look at the drill holes and the surface compared to a good cultured pearl. There is not a one-in-all test I can recommend. Some of this will simply require some 'pearls' of real and fakes for comparison. When you look through a 10x maginfier at the surface of most fakes, the surface might look gritty, uneven, "orange peel" as in paints or even flaked off in small places.
There is a difference difficult to describe. The comparison of the nacre of a pearl to the fake surface is a good indication for use at home, following your teeth, of course. Again, look for the rising or falling surface at the drill holes, like a "painted surface" has sunk into the hole or been pulled up from it.
Even the jewelers are fooled once in a while but not often. Have you seen Mabe pearls? These are the large 'half pearls' often seen in rings and pendants. Some years ago some really good fakes hit the market and were being sold at very competitive prices -- that is, compared to the real ones. Many were sold and bought as the real thing! The testing for these required disassembly of the fake mabes plus solvent and acid tests. A lot of buyers were taken in on this one, likely never to know they bought and later sold fake mabe pearls.
If you get pearls and you are not sure if they are fake or real and you have a hard time to tell them apart, go to you closest jewelry store and talk to them. See if they can run some of the test that I just menched to you. They will be able to tell you if the pearls are fake or not. But like is said it is very hard to tell some of the real ones from the fake ones. You need to be careful when you are shopping around for any kind of jewelry and make sure that you go to a jewelry store that you know are good.