Beef should have a coarse marbling of milky white fat running through it. If the marbling is minimal or if the fat has a brown or yellow tint (a sign of old, dry meat), avoid it. Also avoid meat with large clumps of fat within the flesh. The thin marbling will melt and give the flesh richness and juiciness; the large clumps can be greasy and cause flare-ups.
The flesh should have a rich pink or light cherry appearance. If it has a deep red or other dark color, there's a good chance it came from a dairy cow and the meat will be bland and tough.
The surface should be moist, but not wet or sticky. A cut of meat that has been individually wrapped should not have much liquid in the package. That would indicate that the meat had been frozen and thawed.