There is no evidence to suggest that dentists can tell if someone has performed oral sex. This is a common myth that has been perpetuated for many years, but there is no scientific basis for it. Dentists are trained to look for signs of oral health problems, such as cavities, gum disease, and oral cancer. They are not trained to look for signs of sexual activity.
The myth that dentists can tell if someone has performed oral sex likely stems from the fact that oral sex can sometimes cause minor injuries to the mouth, such as cuts or bruises. However, these injuries are not always visible, and even when they are, they are not specific to oral sex. They could also be caused by other activities, such as eating or brushing your teeth too hard.