Naming conventions of OUs, computers, users, groups.
DNS, network, and DHCP config
GPOs
A list of enterprises hosts
Any trust relationship
Users who have elevated permissions.
AD hardening categories
People
Processes
Technology
Additional AD hardening techniques
Creating an AD snapshot with AD explorer
Use valid domain credentials to login
PingCastle
It is a powerful to evaluates the security posture of an AD environment and provides several different maps and graphs. PingCastle can be a great resource to help you gather one in a nice user-readable map of the domain and it also provides a detailed report of the target domain's security level using a methodology based on a risk assessment framework.