If you want to find the list of leads (previously synced to SFDC) who failed to sync among a particular group of leads, you can filter by following the below steps.
1. Create a new Boolean type field in SFDC namely "Sync Error?" and with the default value as "true".
2. Wait for the field to sync into Marketo.
3. Create a Smart Campaign to select those group of records and the flow steps to change the value of the Boolean field "Sync Error?" to ”False" and then "Sync person to SFDC".
4. Once the campaign has finished running, you will be to pull a report on SFDC on the list of leads who have the value "True" for the field "Sync Error?". It means that the value was not updated to SFDC as the record failed to sync. With this list you will be able to select the records and resolve the sync issue.
You can also create a smart list of leads who were never able to sync with the following filters: