If multiple people get the classified document it is very hard to find out who leaked it unless the recipient testifies who gave it, there is some identifying info in the document (such as a serial number or slightly different copies given to each person) or you catch him in the act. Did Schiff give it? Did another representative? Did a member of any if their staffs? How can you tell?
By the press conferences Schiff gives at the bottom of the stairwell outside the CONgressional SCIF.
He’s done it a number of times. Including during Impeachment.