Hello OT,
I don't think so there is any flag in USERS entity to identify that user is from AD group. When AD authentication happens if user is authenticated successfully then user is created in User table while first time login otherwise it gets updated like last login datetime.
If you create an user before authentication and AD authentication works on Email(username) it may not give any error since its authenticates via email or username and if it finds same email in USER table it will just update it and not create it.