You can simply identify it by checking /etc/resolv.conf.
If there is a line of VxRail Manager IP, it's internal DNS on VxRail Manager.
If there is a line of customer supplied DNS IP, then it's external one.
Thanks naoyuki, but what about from VMware vSphere? I'm checking all possible options but I can't see anything from GUI.
AFAIK, there is no clue in vSphere Client GUI.