As a DBA, I work closely with the Infra guys to manage domain accounts. Now and then, there comes a time when managing Security in my SQL Servers i need to verify if a user or a group is already existing, locked or deleted. To do this without always bothering your Infra teamates (and usually they close the shop earlier than you, the DBA) you need to know by heart these 2 commands that you can run from your command window.
net user /domain <usernameyourelookingup>
net group /domain <domaingroupyourlookingup>
The net user command does just that, it returns to you if the user name is existing in the domain and its status.
The net group command returns to you the current members of that group.
This 2 commands should be written at the end of your fingertips, you will always need to this when troubleshooting domain issues within your Security.