Where has a custom field been used?
I can think of a way, but you need to know how to run SQL on your CiviCRM database.


Find out the ID number of the custom field.
Run the following SQL.  Change the "7" in custom_7 to the ID number of the custom field:




SELECT cug.id, cug.title 
FROM civicrm_uf_field cuf 
JOIN civicrm_uf_group cug ON cuf.uf_group_id = cug.id 
WHERE field_name = 'custom_7';


It will give you back the title of any profiles that use the field.

Of course, if you're using Webform-CiviCRM or Caldera Forms-CiviCRM integration, this won't catch those!