The system can, after a roster upload, provide you with a list of Unique IDs in the roster CSV file that did not match with Unique IDs of students in the course. To see how to upload a roster, read the help article How can I verify the students enrolled in my course are the same roster of students provided by my institution's online course management system?.
View missing UniqueID Roster
Clicking Missing UniqueID Roster will allow you to open or save a text file (.txt) containing a comma-delimited list of Unique IDs found in the roster file that did not match with Unique IDs on the system. In our simple example, there are four non-matching Unique IDs (one being the column header).