Sharks lose thousands of teeth while they are alive, as they are constantly growing new ones. Their teeth, being tougher than their cartilaginous skeletons, remain in the environment long after they drop out or the shark dies, and some fossilize. This means there is no shortage of real old and new shark teeth in the world. However, fake ones do exist. In some cases, such as new-looking teeth from endangered species like the great white shark, it is in fact better to buy fakes. In other situations you might be disappointed to discover that the “fossilized shark tooth” you bought is actually made of plastic. Telling the difference is straightforward in most cases. Shark teeth are not so valuable that many sellers will spend time or money on creating high quality fakes. Most fake shark teeth are openly sold as such, and real ones aren't normally worth much more.
Visit a museum with an extensive collection of shark teeth. This is the best way to get a good idea of what real ones look like.
Look up the kind of shark tooth you are most interested in buying and their distinguishing features, such as general shape and distinctive grooves. Guides are available online or for sale in shark tooth hotspots. A tooth that doesn’t look anything like the pictures may be a fake, or more likely a tooth misidentified by the seller.
Touch the tooth you are unsure about. Crude plastic imitations may feel slightly warm, while the stone of fossils feels cold.
Examine the tooth you want to check with a magnifying glass. Real teeth, especially fossilized ones, are full of imperfections, chips and irregularities. A very smooth, perfect tooth may be a fake.
Take the tooth to an expert at a local museum or university to get a definitive answer and identification. If you bought the tooth from a physical shop, rather than online, not only is it most likely real, but you can return it if it does turn out to be a fake.