"Do dentists know if you've given head" is a question that has been asked by many people, but there is no scientific evidence to support the claim that dentists can tell if someone has performed oral sex.
There are a number of reasons why this claim is likely to be false. First, the human mouth is a complex and dynamic organ, and there are many factors that can affect its appearance, including genetics, diet, and oral hygiene habits. Second, there is no specific set of physical signs that are universally associated with oral sex. Third, even if there were a specific set of physical signs that were associated with oral sex, it is unlikely that dentists would be able to reliably identify them, as they are not trained to do so.