According to an article published by The Pearl Market, most inherited pearls turn out to be imitations because in past generations most people couldn't afford to buy real pearls. In addition, most vintage pearls aren't worth a lot of money even if they are real. The nacre, or luster, of pearls can wear off if they've been worn or handled for several decades and advances in the technology of culturing pearls means that pearls being produced today may be much more desirable than those produced a few decades ago. However, some vintage pearls have maintained their value and there are a few simple ways to determine their quality.
Measure your pearls. If you know the era your pearls are from, measuring them can help determine if they are real or fake. From 1950 through the 1970s an average pearl was 4mm to 6mm in size. If your vintage pearls are from this era and they are much bigger that this it indicates that either the pearls are imitations or they are real and were owned by someone who was very wealthy.
Search for imperfections. Whether cultured or natural, real pearls have some imperfections or blemishes. If your vintage pearls are perfectly round and 100% free of blemishes they are likely imitations.
Inspect the clasp if your vintage pearls are still strung on a necklace or bracelet. The clasp may have a makers mark or 14k stamp showing that it's made of 14 karat gold. A cheap looking, silver or gold-plated clasp indicates the pearls were not valued enough to warrant fine closures.
Take your vintage pearls to an accredited appraiser. Accredited appraisers can be found online, in the phone book or by asking your jeweler for a recommendation.