How do I identify who is a manager in HCM?

Summary:

I am trying to find out how to identify when someone becomes a manager in the system. There is no change to their work relationship in HCM, its their direct reports record that will change.

Content (required):

We want to be able to run reports and generate an alert when a person becomes a manager for the first time, however as there is no change to that persons record - how can we do that? Its not always linked to a promotion, it could be because of another manager leaving. There is a flag on the managers record, but it has to be manually changed as far as I am aware, does anyone have any idea how I can accomplish the identification of these people, using the assumption that I dont have someone to keep this