You can decipher whether a pearl is real by gently rubbing it on the front of your tooth. A natural pearl will feel gritty and if it is fake, it will feel lighter and smooth.
Colouring is another indication of whether a pearl is natural. If antique pearl rings and jewellery are very slightly yellow, they are likely to be real, as natural pearls are subject to a change in colour over time. The best way to be sure whether a pearl is natural is to go to a certified jeweller.