The rule of thumb is that every ounce of meat in your turkey must reach a minimum of 165 degrees F. (75 degrees C.). Measure this temperature in the thickest part of the breast and the thigh, just above the drumstick. If you don't reach this temperature you run the risk of making your guests very ill.
By the time the breast meat reaches this magic temperature, the dark meat will be 180 F. (82 degrees C.). It is good to measure this temperature as well to be on the safe side. Measure the temperature of the dark meat in the center of the thigh. This is a good thing since the dark meat should be cooked to a higher temperature than the white meat to achieve perfection.
What you also want to look for is that all juices running from the meat everywhere are clear unless you have smoked the turkey (smoked meats remain pink due to nitrates in the smoke).
The temperature difference between the dark and white meat can mean that the dark meat might get overcooked.