It's not absolutely necessary to utilize a thermometer to check the internal or external heat temperature of the meatloaf. However, it can save you a lot of stress and uncertainty during the cooking process. You will know the difference when using one, especially because you'll want to prepare this dish more than once. It's better for the long term to make a habit of checking the temperature to make sure it doesn't need cooking. In some cases, ovens can vary over time in their performance so it's better to be safe than sorry in this instance.