On the left side of your screen you should be able to see a list of all of your "followers" (people who are following you), whether they follow daily or not. If one of yours is missing from the list, then that's a pretty good indication that he/she unfollowed you! However, there are some cases where people delete and re–add their accounts so next time you mention them or send a snap to the recently added account make sure it was actually deleted and not just a case of error.