The new lender will have that knowledge and the title company will be able to get the final number when the new loan closes.
This is usually the job of the closing agent or your lender. You sign a form permitting them to obtain this type of information and they process it. Typically all such requests must come in writing and are for a specific date with a daily interest cost add on if it closes early or later than projected.
Did your lender request you get this or the closing agent?