We just recently ran into an issue wherein the sync with SFDC went down. Due to that happening, we had a number of leads that were not added to SFDC campaigns. We ended up going through activity logs on each lead that should have been created manually.
Therefor the idea is to create an smart filter that can be used inside the activty logs. Ie. filter on "SFDC Lead Updates to SFDC". Additionally, the filter would have parameters for if it was successful or in an error.