The best way to determine if a casserole is done is by testing the internal temperature of the food with an instant-read thermometer. For most casseroles, you should aim for an internal temperature of 165 degrees Fahrenheit or higher. If you don't have an instant-read thermometer, you can test your casserole by inserting a knife or fork into the center and making sure that it comes out hot. You can also check to see if any cheese added to the top is melted and beginning to brown. This indicates that the casserole has cooked through sufficiently. Finally, make sure that there is no liquid left in the dish when it is removed from the oven. If there is still liquid present, you may need to put the dish back in the oven for a few extra minutes.