The only way to be absolutely sure that meat has been cooked to your liking is to use a meat thermometer. The thermometer should be inserted into the thickest part of the joint, without touching any bones, just before the end of the cooking time. When cooking beef and lamb, the meat will be rare when the temperature reaches 60C, medium rare when it is 71C and well done when the temperature reads 80C. Pork and veal should only be served well done, which means that the temperature should reach 75C for pork, and 71C for veal.
If you do not have a meat thermometer, you can insert a metal skewer into the thickest part of the cooked joint and wait for 30 seconds. If the skewer feels cold to the touch when it is withdrawn, the meat is not done; if it feels warm the meat is rare, and if it is hot the meat is well done.
It is wise to take the resting time into consideration when you are testing for doneness because the meat will continue to cook while it is sitting. This is more important for a piece of meat that you wish to serve rare or medium and less crucial if the meat is going to be eaten when it is well done.
** Cooking Thermometers