From what I have heard, there are usually rounds of acceptances if many students are expected to decline their offers. Then, they move to the next batch and so on until they fill the 5-7 seats they have available by the April 15th deadline. Some schools will have an internal waitlist where they won't notify you that you are on it. Other programs may have an official waitlist, which means they would notify you about it. Some programs might reject you, but not update your status or notify you until the deadline or after. It seems like there's no way of knowing whether you are on an internal waitlist as a "back-up," or if your application was thrown into the rejection pile. You can always solicit and see if you can gather any information...