I recommend using a meat thermometer. Place the thermometer so the bulb
is in the thickest part of the thigh, the largest muscle. Make sure
it isn't touching the bone, which conducts heat differently and will
register a lower temperature than the muscle. When you cook a stuffed
bird, the temperature in the thigh should reach 180°F (82°C).
At that temperature, the stuffing should measure 165°F (74°C),
rendering it safe to eat. Cook unstuffed turkey to 170°F (77°C).
As with any large piece of meat, remember that cooking continues after
it's removed from the oven, so take a whole turkey from the oven when
the thermometer is a few degrees shy of the desired final temperature,
and let the bird stand for a few minutes. This completes the cooking,
allows the juices to settle, and makes carving easier. Happy Thanksgiving!