They receive a notification in their ORCID inbox. Whenever there is a change in data on an ORCID record, the user receives a notification with basic information about the change, including the name of the client performing the update, date of change, and the item in the ORCID record that was updated. See our user Knowledge Base for more about notifications.
ORCID records on the sandbox also have functioning inboxes. We recommend regularly looking through the inboxes of your testing ORCID records when planning how your systems will schedule data updates.