A book of photos containing examples of work by an artist.
The quickest and easiest way to judge the quality of an artist (tattoo or body piercing or scarification) is to ask to see their portfolio. This should contain many photos of their work. Look for both healed and fresh photos, as well as multiple photos of the same type of procedure showing consistent quality throughout.
It goes without saying that some less reputable artists will fill their portfolio with photos cut from magazines and printed from websites. These "artists" should of course be avoided.