The following knowledgebase will show you how to find a client’s actual domain name on which client has logged on successfully.
By typing Net Config Workstation you can see the domain name. The output will be:
The “Logon domain” in above output shows the domain name on which client has logged on successfully. This domain controller of this domain has logged on client successfully. This also shows that there is trust relationship between child and parent domains.
You can also search the following registry key to check which DNS Domain name was used by the Group Policy:
You can see the DCName in the right pane which contains the DNS Domain name of client machine.