The phrase "can the dentist tell if you've given head" is a slang expression used to question whether or not a dentist can determine if someone has engaged in oral sex. The answer to this question is no, dentists cannot tell if someone has given head. There is no scientific evidence to support this claim, and dentists are not trained to look for signs of oral sex during an exam.
This phrase is often used in a joking or humorous way, but it can also be a source of anxiety for some people. If you are concerned about this issue, it is important to talk to your dentist. They can help to reassure you and answer any questions you may have.