You do not need to know all about pearls to know the difference between fresh water and cultured pearls. But there are some differences it is good to know about.
There are mainly four types of pearls:
Akoya, Freshwater, South Sea and Tahitian. Each pearl type is unique with the own color, shape and size.
Freshwater pearls are also known as Biwa pearls as they originated from lake Biwa in Japan are also now mostly available from China.
The Akoya pearls tend to be the most popular as they are very uniform and are usually the same round shape and size so are ideal for necklaces. They are cultured from saltwater mollusks from Japan and China. Akoya pearls are popular for their luster and beauty. They tend to be the most expensive also. However, most cultured pearls do not have a certificate especially when set in jewelry so one needs to have some understanding and familiarity with pearls to ensure that the quality is acceptable.
Freshwater pearls on the other hand are made with cultured freshwater pearls mostly from China and made in a freshwater mollusk in a lake, pond or river. They are similar in look to the Akoya pearls, but are less expensive as they are usually smaller and less symmetrical.
Southsea pearls are a larger pearl that come in a range of light colors. Southsea pearls are somewhat harder to cultivate and therefore tend to be more expensive.
Black Tahitian pearls are darker and larger. They come from the South Seas, such as Tahiti.
Freshwater pearls are ideal if one is on a budget as they are less expensive. They are not so round and are smaller than the Akoya so not so suitable for necklaces but certainly fine for rings, pendants and broaches etc.
Provided one uses some common sense and practices some due diligence knowing the difference between fresh water and cultured pearls is not a difficult task at all.