How do you tell real freshwater pearls from fake ones?

Touch Test

An old wives' tale says that if you hold real pearls in your hand, they will be cool to the touch for several seconds before warming up. Genuine pearls tend to warm with contact to the skin much faster than glass pearls. Resin or plastic pearls tend to feel somewhat warm upon first contact. This however is not a sure-fire method for checking authenticity.

The Tooth Test

Rub the pearls lightly along the biting edge of your upper front teeth. If they feel slightly rough, sandy or gritty, it's likely they are cultured or natural pearls because of the layers of nacre that have formed over time. If they feel smooth or glassy, they are probably imitations. You will also get the same result (rough, sandy, or gritty) if two pearls are rubbing together.