The rumor is that Gen. McMaster uncovered Rice’s guilt by examining the log of those who had signed in to see the sensitive material.
McMaster notified Chairman Nunes, who then went to notify Trump.
As I understand it, this step should not have been necessary. If Susan Rice is the person who authorized the unmasking that act would require her signature on some document that should be accessible to the Trump Administration. AFAIK, that is Standard Operating Procedure in the IC.