The best way to check to see if a prime rib is done is with the use of a meat thermometer. Meat thermometers can be purchased where kitchen supplies are sold.
If rare prime rib is desired, the internal temperature should be between 120 and 125 degrees Fahrenheit. If a medium-rare doneness is desired the temperature should reach an internal reading of 130 to 135 degrees F. Well-done prime rib needs to reach 145 degrees F.
It is not recommended that prime rib exceeds temperatures of 155 degrees F. Best results are also achieved if the prime rib is allowed to sit out at room temperature for at least 1 to 2 hours before cooking.