Giles has the mark of Eyghon tattooed on his left inner-upper arm. The story behind the tattoo: When he was in his early twenties, Giles and a bunch of his friends became interested in demons and conjuring them into each other's bodies. It was the ultimate high for them and they tattooed themselves with the mark to make it easier for Eyghon to inhabit their bodies. Eventually, Eyghon took over the body of one of Giles' friends and they were forced to kill him. Ethan Rayne was also a part of this group. They eventually disbanded and Eyghon hadn't been heard of since then, until a friend of Giles' came to Sunnydale, infected with Eyghon. Angel was finally able to kill Eyghon by strangling it and having the demon inside himself fight Eyghon.
     
  www.imdb.com reports that Anthony does in fact have a tattoo on his back. It's located on his left shoulderblade. (thanks to Wendy)