Take a look at a sample of systems that may be defined as Tier 0 systems.
To begin the identification of Tier 0 assets, focus on these three subjects:
Once the list of assets is created, investigate each listed system. Make a current list of administrators for each.
Examples to identify administrators:
VMware - export the administrator list. Which VMware administrator(s) have access to the domain controller vms.
Provisioning system: provisioning account name. Users with access to the provisioning account
If you need more assistance with identifying access across your organization, please reach out.