The easiest and safest way to check if pork tenderloin is done cooking is to use a meat thermometer to check the temperature of the meat. Pork tenderloin should needs to reach an internal temperature of 71.1 degrees C to be safe. Looking at the colour of the meat doesn't always work. Your pork loin could still have some pink to it but be safe to eat, or it could be a solid white but not fully cooked to a safe temperature. As a general rule, pork tenderloin roasts at a temperature of 162 degrees C for about 35 minutes per pound of meat.
Prepare and cook the pork tenderloin according to your recipe.
Pull the oven rack out slightly so that you can check the temperature of the tenderloin without removing it from the oven.
Insert the meat thermometer into the thickest part of the meat and check the temperature of the tenderloin. If the meat is not done, continue cooking until the temperature reaches 71.1 degrees C.
Remove the pork tenderloin from the oven. Allow it to sit for a few minutes so the juices settle in the meat. Serve while still hot.