You can view the sync status of any integration in the portal itself by going to the Linked Accounts section of the Dashboard. Click on the Linked Account you'd like to check and you will see the status surfaced for each common model.
Additionally, you can run a GET request for /sync-status with the appropriate base URL, which will return the statuses of your integrations! You can view a sample response in our Developer Docs.
Tips and Recommendations:
Call the /sync-status endpoint before pulling data to make sure you are getting complete data from Merge!