Can I cook a pre-made meatloaf from frozen? And if so, what temp, covered or not covered, and how long - how do I know when it is done?
Yes, you can cook a meatloaf from frozen. At 350F (175C), it will take about 1.5 times the length of time it would take to cook a non-frozen meatloaf of the same size. You will get more of a crust cooking a meatloaf from frozen, so consider that while considering whether to cook the loaf in a pan or not, also whether to glaze it. The crust will be somewhat inhibited by covering for the first half of cooking, so consider that as well, that's a matter of personal preference. If you use a glaze, let the meat cook for a half an hour or so before you apply the glaze. If you glaze before you put it in the oven, you'll risk burning the glaze.
Use a thermometer, either instant read or leave-in - using both is even better. The meatloaf will be done when the thermometer(s) reading the very center of the loaf reads at least 160F (71C). Let it rest for a few minutes before serving.