If there is no field on the table to identify the last time a record was changed, how do they expect you to define the delta.

Your options:

Full snapshot of the table for yesterday and today and compare records based on primary key - slow and takes a lot of space if the tables are large.

Set a trigger on the source table to write to a changed record log table and use this as the source for the delta extract

If you know that records are only modified within (say) 5 days of their creation date you could pull all records that were created in the last 5 days and treat that as a delta and let the 3rd party handle the SCD issue.