In the USA, the US Department of Agriculture has established a grading system for beef. Therefore, when you purchase a package of meat, it will have a shield or sticker from the USDA, which signifies the grade.  When I buy filets, I always look for either prime or choice. Steaks labeled prime or choice are high quality, and they will be tender, juicy, and flavorful. The main difference is in the amount of marbling they have. Choice has less marbling than prime and, consequently, may not be quite as juicy or tender.  Because filet mignon is so expensive, ask your butcher if you are still unsure how to pick out a steak. They are usually happy to help and can keep you from making an expensive mistake.