Rub the pearl against your teeth. You may be able to tell this way as real pearls are supposedly rough while fakes are smooth. This is not the most reliable method as you can find smooth real pearls and rough fakes.
Hold the pearl up to the sunlight or very bright indoor lighting. You can check for variations in the pearl’s color and tone. If it’s perfect in its color and tone, it’s very likely fake.
Look at the pearl through the magnifying glass. You should be able to see the ridges and irregularities of a real pearl, or the grainy smoothness of a fake.
Weigh the pearl. Real ones are usually heavier than fakes.
Test the warmth of the pearl. Resin and plastic pearls are often warm on first contact. Glass pearls take longer to warm in your hand than real ones.
Check the drill holes. Real pearls are drilled as small as possible to maintain their value, so fakes will often have larger holes. Also, the pearl surface, or nacre, will often flake off around the holes of fakes.
Check the pearl’s shape. If it’s a perfect sphere, it’s likely a fake.