Typically, frozen store-bought meatballs are fully cooked so then the only concern is that they are heated through. Which means you can use a fork to test it they're done. If the fork pierces the meatball easily, the meatballs are warmed through. If your meatballs are starting out raw, or you are concerned about the internal temperature, use an instant read the meat thermometer and make sure it reads 165° when inserted into the center of a meatball.