Professional chefs may press a roast with their fingers to determine if the meat is rare, medium or well done, but home cooks need a less subjective, more reliable method--namely, a meat thermometer. See the "Timetable for Roasting Beef" in Beef & Veal for thermometer readings and cooking times.
A conventional meat thermometer stays in the meat throughout cooking. Some people think this lets too much juice escape. An inexpensive instant-read thermometer is an easy alternative. A little before you think the roast is nearly done, insert the instant-read thermometer’s probe three inches into the center of the meat; make sure the tip doesn’t rest on fat or bone. After 30 seconds, the thermometer renders an accurate temperature. Remove the thermometer; it should not remain in the oven. Remove the roast from the oven when it reaches about 5°F. below the temperature you want; the roast continues to cook as it rests on the counter.