Add ability to run report for contacts that don't sync in Salesforce

Issue -

It appears some of our users accounts are not syncing with Salesforce. I am thinking it is because of one of the following reasons - 

1. They aren’t using the email associated with SF
2. They aren’t a contact in Salesforce
3. They have an updated email address and we should update in Salesforce

Ask - 

We’d like to be able to run a report that captures who is not syncing to Salesforce so we can use that data to update records, etc.