The first rule for roasting a turkey is that if your bird comes with one of these pop-up timers, throw it away. It's really not a reliable indicator. By the time it pops up, your turkey may be as tough as shoe leather. Instead, use an instant-read probe thermometer; either an analog or a digital one is fine.
Start checking for doneness on the early side and insert the thermometer in the thickest part of the turkey's thigh. Be sure you're not hitting the bone, which will give you an inaccurate reading. Give it a minute to settle on a temperature. What you're looking for is a reading of 170°F in the thigh.
It's also a good idea to check both thighs, because if your turkey has to fit sideways in your oven, you might find that one side cooks faster than the other because of hot spots in the oven.
If you've stuffed your turkey, you also have to take the temperature of the stuffing to make sure it's fully cooked as well. Stick the thermometer probe right into the center of the stuffing; now you want a reading of 165°F.
If your turkey is fully cooked, but the stuffing is not, scoop the stuffing into a baking pan and continue baking it until it reaches 165°F.
Meanwhile, take the turkey out of the roasting pan, place it onto a cutting board, cover it with a big piece of foil, and let it rest for 30 to 40 minutes. Resting a turkey after roasting lets the juices redistribute so the meat is equally juicy in the leg and the breast.
Videography by Gary Junken and Michael Dobsevage; editing by Cari Delahanty