The question of "how can a dentist tell if you've given head" is a common one, and there are a few reasons why. First, certain oral sex practices can cause physical changes to the mouth, such as bruising or swelling of the lips, tongue, or gums. Additionally, some sexually transmitted infections (STIs) can be transmitted through oral sex, and a dentist may be able to identify signs of an STI during a routine exam.
It is important to note that not all dentists will be able to tell if you have given head, and there is no surefire way to hide any physical changes that may have occurred. However, if you are concerned about the possibility of a dentist being able to tell, it is important to practice safe sex and get regular STI screenings.