Since fields are global in your organization, it can be useful to look at all of the fields that have been defined.
You can get this overview by accessing the "Manage Organization Fields" dialog in the "Organization Settings" menu (only visible to Organization Admins):
By hovering the workspace count, you can see which workspaces are using the field:
The view allows you to delete fields across all workspaces. When you click Delete it will first show you a warning and list where the field is in use: