The ideal thickness for a steak should be 2.5cm thick or even thicker. It is actually easier to perfectly cook a thicker steak compared to a thin steak. If you buy a thin steak you run the risk of overcooking the steak, by the time the outside has nice sear marks it will be well done in the centre.
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. Avoid beef that is dull in colour or is discoloured.
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.