Through testing we've noticed a couple of times where syncs back to SalesForce have resulted in errors. Either by not having a high enough write level, or fields missings.
Whilst we think we've found them all, I would rather be notified of any problems, so they can be fixed sooner rather than later.
Looking at the activity log for a user I can see Sync Lead Updates to SFDC entries and these have the relevant error messages when things do go wrong. However I can't find a way to search for all of these logs.
Other threads for looking at the full activity log suggest using the SOAP API.
Running a getLeadActivity() call I can see succesful Sync Lead Update messages, but not those of with errors, but even then I would need to loop through all the user who might have been updated.
Running a getLeadChanges() I can't see any option to filter against the Sync Lead Updates, only 7 activty filters have SFDC in them and those are to do with activity on SalesForce.