Hi Michelle,
I think it depends on how fields of SFDC are mapped with Marketo fields.
The alternative filter could be, 'Lead Source.' In the dropdown you will be able to see all the lead sources. You might want to check which lead sources are configured with your SFDC and which are not and then adjust the filter settings to eliminate all the sources from SFDC.
Also, if you have uplaoded the leads list directly to marketo via a CSV file or manually created the the lead with no mention of lead source then you can use the same filter, 'Lead Source' and the source is Empty'. But again this might not be a good solution if you have selected the Lead source in the uploaded file which is similar to what is configured in SFDC.
Hope this is of some help to some extent.
Thanks & Regards,
Vaishali