The phrase "can dentists really tell if you've given head" refers to a popular but false belief that dentists can detect signs of oral sex during a dental examination. This belief is likely based on the assumption that oral sex can cause physical changes to the mouth, such as bruising or swelling. However, there is no scientific evidence to support this claim.
Dentists are trained to examine the mouth for signs of disease and other problems. They use a variety of tools, including a mirror, explorer, and X-rays, to visualize the teeth, gums, and other oral structures. While dentists may be able to detect some signs of oral trauma, such as a cut or abrasion, they cannot reliably determine whether or not someone has engaged in oral sex.