The questions above and similar questions can be easily answered when using the Administrative Tool "Active Directory Users and Computers".
In "Active Directory Users and Computers" (ADUC) connect to the domain, highlight the domain (root) or any container in left side pane, right-click and select find and when a new Window opens select the "Advanced" tab.
Click on Field - User - and select eg. Custom Attribute 14.
Click on Condition and select eg. Not Present
If you click on "Find Now" and confirm the new window with "yes" you will receive a report. In our example, it will show you all objects which do not have the Custom Attribute 14 populated.