Using a meat thermometer
There are two main kinds of meat thermometer - instant read and ovenproof.
The ovenproof variety are designed to be inserted into a joint of meat and left there will it roasts. It needs to be placed into the thickest part of the meat, not touching a bone, and preferably so that you can see the gauge through the oven door. This way you won't need to open the oven to check the temperature it has reached.
Some feature a marker on the edge, enabling you to see clearly when the pointer reaches the optimum temperature.
Do remember that when you take an ovenproof meat thermometer out it will be HOT, so use an oven glove!
The instant read thermometer is not left in the oven but used to quickly test for doneness, and gives a digital readout in seconds. It can also be used for candy making and fried foods.