I've yet to come across a credible fake. They do exist, but Megalodon teeth are common enough in the fossil industry that they're generally not worth faking. It really depends on provenance. For instance, North Carolina Megs are a dime a dozen. Commercial divers find them everyday, and tours leaving from Wilmington will even take you 30 miles off the coast to dive for them. Indonesian Megs used to be rarer, but not so much in more recent years.

With Megs there are other issues to be concerned with besides outright fakes. I collect these from localities around the world. For me, the biggest concern is faked provenance. I have one from Morocco. Those are really rare. I did have to do a lot of work to understand how likely it was that the tooth being sold was actually from Morocco.

On the other hand, the general buyer would be more concerned with repair or restoration work, which is when legitimate Megalodon teeth are repaired with glue or restored with epoxy and paint to increase their value. This should always be declared, but with more sketchy sellers sometimes it's not. Again though, NC Megs and smaller teeth from the Southeast are so common that restoration on them isn't that common.