You can view the sync failure from Marketo to SFDC when you click on a lead's Activity log. For instance one of the big sync problems we had was a phone, mobile and fax formatting restriction on SFDC, which prevented leads from syncing from Marketo to SFDC because the formatting has to have a +1. Saw that on the activity log and that's how I found out that we had a problem. It was since removed from the user and all is working fine now.