How do I know whether my filling is lost versus my tooth breaking away?

Hard to tell . Seeing a dentist and getting an xray are the best ways to determine if it is a problem with your tooth or the filling. Please see a dentist quickly before you have pain or the tooth fractures causing a more expensive problem to fix.
Need to visualize. You may not be able to tell. But your dentist can. Make an appointment to have that tooth examined, because either way, it needs to be restored. The longer you wait the deeper the defect will become. Hope this info helps..
Could be both. Unless you can fully visualize it and actually understand what you are looking at, it is best to see a dentist. In either case youn eed to have it treated. Delaying treatment will only make a bad situation worse.