Safe temperature for poultry is 165F, cooking it above will make the meat tougher (brining turkey beforehand will help the meat from drying)
165F=73.9C is the "US government official" safe poultry temperature - since 2006. Before it was 180F :). It is often said that poultry meat should be cooked till the meat is no longer pink. This is incorrect, most important is the final temperature. The "no-pink" indicator is foolproof, but meat will end up overcooked, so using thermometer is the way to go.
Thickest parts will take longer to cook, so as Rich recommended - stick thermometer into the thickest part, but make sure not touch the bones as they conduct heat better and can make reading incorrect.
Thermometer can be inserted through the bag in the beginning of cooking.
Also, when you take the turkey out of the oven it will continue cooking and internal temp will rise another 5-15 degrees because outer layers of meat will be hotter, so if you take the it out at 160 or even 155 and cover, the cooking will stop just at right temperature. This is also called "carryover cooking".
I have an electronic readout thermometer - polder i believe - that has a thermocouple probe connected to the display unit with about 4ft metal-cased wire - so you can have the probe inside the oven and the display outside. With that display you can set alarm at any temperature.
The only disadvantage of electronic thermometers - they are not water-proof. Only the probe part which is about 5 inches long can be immersed into the liquid. Remembering this will help you use this thermometer for a very long time.
Dial thermometers that can go inside the oven are ok, but it's not always easy to see the display inside the oven, they are not as accurate and obviously have no alarm.