There are any number of methods for determining the doneness of a piece of meat. Some would have you press the meat with a finger or utensil, gauging how cooked it is by the firmness. Others work from rules of thumb about temperature and cooking time. Ultimately, though, no traditional technique works as well as a thermometer for determining the doneness of a roast.
Select a thermometer to use with your roast. Traditional meat thermometers are inserted into the roast and left there, making it necessary to open the oven and lose heat to check the temperature. Probe thermometers connect to a digital readout, so you don't need to open the oven. Instant-read thermometers can't stay in the roast, so the oven must also be opened to take a temperature with the instant-read variety.
Check the temperature near the end of the cooking time, using whichever type of thermometer you have. Judge your estimated cooking time by the recipe or on a basis of 20 minutes per pound of roast.
Remove the roast from the oven when it reads 5 to 10 degrees lower than the desired end temperature, since it will continue to cook after it comes out of the oven. Very large restaurant-style roasts can increase as much as 20 degrees after being removed from the oven.
Decide what level of doneness you desire. Beef roast is rare at 120 to 130 degrees Fahrenheit, medium-rare at 130 to 135, medium at 135 to 145 and well-done at 160 degrees Fahrenheit.