If you have net new leads that are connected to existing SFDC Accounts, I assume these are all syncing down from SFDC to Marketo as Contacts. Please correct me if I'm wrong and there is different process you have.
If the above statement is true, then it should be pretty easy to distinguish between the two.
All new leads in Marketo with lead creation source of SFDC and SFDC Type of Contact would mean they are new Marketo leads connected to an existing SFDC account. The rest will simply be Marketo net new leads.
Inside sales is also using leads/contacts. Their leads sync with Marketo, they are turned into contacts when an account is associated with them. Sales also using leads seems to muddy the water.