Identify Marketo leads not in Salesforce?

I'm trying to identify leads in Marketo that may not currently be synced to Salesforce (this is probably incredibly basic, but I can't seem to find the exact answer here so far!).

Any suggestions on the key defining fields that I could use to identify these leads? There's a filter for SFDC Account Number, but not a lead-level attribute ID number. Would "SFDC type is = Empty" work?

Thanks for your help!