I have a process that works partially using native SFDC and Marketo functionality. My SFDC admin has a field that calls the 18 character ID of each lead and contact, he's shared that to my Marketo instance. Then I just have a batch campaign that runs nightly that says "Lead 18 character ID missing AND Contact 18 character ID missing". If both are empty, the lead didn't sync for some reason and I can investigate and manually sync it.
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